Add plugin CLI argument (#6064)

* Add plugin CLI argument

While working on supporting CustomValues in Plugins I stumbled upon the
test utilities defined in [nu-test-support][nu-test-support]
and thought these will come in handy, but they end up being outdated.
They haven't been used or since engine-q's was merged, so they are
currently using the old way engine-q handled plugins, where it would
just look into a specific folder for plugins and call them without
signatures or registration. While fixing that I realized that there is
currently no way to tell nushell to load and save signatures into a
specific path, and so those integration tests could end up potentially
conflicting with each other and with the local plugins the person
running them is using.

So this adds a new CLI argument to specify where to store and load
plugin signatures from

I am not super sure of the way I implemented this, mainly
I was a bit confused about the distinction between
[src/config_files.rs][src/config_files.rs] and
[crates/nu-cli/src/config_files.rs][crates/nu-cli/src/config_files.rs].
Should I be moving the plugin loading function from the `nu-cli` one to
the root one?

[nu-test-support]: 9d0be7d96f/crates/nu-test-support/src/macros.rs (L106)
[src/config_files.rs]: 9d0be7d96f/src/config_files.rs
[crates/nu-cli/src/config_files.rs]: 9d0be7d96f/crates/nu-cli/src/config_files.rs

* Gate new CLI option behind plugin feature

* Rename option to plugin-config

src/main.rs

@@ -103,6 +103,8 @@ fn main() -> Result<()> {
                     args.next().map(|a| escape_quote_string(&a))
                 }
                 "--config" | "--env-config" => args.next().map(|a| escape_quote_string(&a)),
+                #[cfg(feature = "plugin")]
+                "--plugin-config" => args.next().map(|a| escape_quote_string(&a)),
                 "--log-level" | "--testbin" | "--threads" | "-t" => args.next(),
                 _ => None,
             };
@@ -204,6 +206,7 @@ fn main() -> Result<()> {
                 read_plugin_file(
                     &mut engine_state,
                     &mut stack,
+                    binary_args.plugin_file,
                     NUSHELL_FOLDER,
                     is_perf_true(),
                 );
@@ -256,6 +259,7 @@ fn main() -> Result<()> {
                 read_plugin_file(
                     &mut engine_state,
                     &mut stack,
+                    binary_args.plugin_file,
                     NUSHELL_FOLDER,
                     is_perf_true(),
                 );
@@ -304,6 +308,8 @@ fn main() -> Result<()> {
                 setup_config(
                     &mut engine_state,
                     &mut stack,
+                    #[cfg(feature = "plugin")]
+                    binary_args.plugin_file,
                     binary_args.config_file,
                     binary_args.env_file,
                     binary_args.login_shell.is_some(),
@@ -329,12 +335,19 @@ fn main() -> Result<()> {
 fn setup_config(
     engine_state: &mut EngineState,
     stack: &mut Stack,
+    #[cfg(feature = "plugin")] plugin_file: Option<Spanned<String>>,
     config_file: Option<Spanned<String>>,
     env_file: Option<Spanned<String>>,
     is_login_shell: bool,
 ) {
     #[cfg(feature = "plugin")]
-    read_plugin_file(engine_state, stack, NUSHELL_FOLDER, is_perf_true());
+    read_plugin_file(
+        engine_state,
+        stack,
+        plugin_file,
+        NUSHELL_FOLDER,
+        is_perf_true(),
+    );
 
     if is_perf_true() {
         info!("read_config_file {}:{}:{}", file!(), line!(), column!());
@@ -398,6 +411,8 @@ fn parse_commandline_args(
             let commands: Option<Expression> = call.get_flag_expr("commands");
             let testbin: Option<Expression> = call.get_flag_expr("testbin");
             let perf = call.has_flag("perf");
+            #[cfg(feature = "plugin")]
+            let plugin_file: Option<Expression> = call.get_flag_expr("plugin-config");
             let config_file: Option<Expression> = call.get_flag_expr("config");
             let env_file: Option<Expression> = call.get_flag_expr("env-config");
             let log_level: Option<Expression> = call.get_flag_expr("log-level");
@@ -425,6 +440,8 @@ fn parse_commandline_args(
 
             let commands = extract_contents(commands)?;
             let testbin = extract_contents(testbin)?;
+            #[cfg(feature = "plugin")]
+            let plugin_file = extract_contents(plugin_file)?;
             let config_file = extract_contents(config_file)?;
             let env_file = extract_contents(env_file)?;
             let log_level = extract_contents(log_level)?;
@@ -455,6 +472,8 @@ fn parse_commandline_args(
                 interactive_shell,
                 commands,
                 testbin,
+                #[cfg(feature = "plugin")]
+                plugin_file,
                 config_file,
                 env_file,
                 log_level,
@@ -478,6 +497,8 @@ struct NushellCliArgs {
     interactive_shell: Option<Spanned<String>>,
     commands: Option<Spanned<String>>,
     testbin: Option<Spanned<String>>,
+    #[cfg(feature = "plugin")]
+    plugin_file: Option<Spanned<String>>,
     config_file: Option<Spanned<String>>,
     env_file: Option<Spanned<String>>,
     log_level: Option<Spanned<String>>,
@@ -495,7 +516,7 @@ impl Command for Nu {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("nu")
+        let signature = Signature::build("nu")
             .usage("The nushell language and shell.")
             .switch("stdin", "redirect the stdin", None)
             .switch("login", "start as a login shell", Some('l'))
@@ -558,7 +579,22 @@ impl Command for Nu {
                 SyntaxShape::String,
                 "parameters to the script file",
             )
-            .category(Category::System)
+            .category(Category::System);
+
+        #[cfg(feature = "plugin")]
+        {
+            signature.named(
+                "plugin-config",
+                SyntaxShape::String,
+                "start with an alternate plugin signature file",
+                None,
+            )
+        }
+
+        #[cfg(not(feature = "plugin"))]
+        {
+            signature
+        }
     }
 
     fn usage(&self) -> &str {