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and thought these will come in handy, but they end up being outdated.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

crates/nu-cli/src/config_files.rs

@@ -1,7 +1,13 @@
 use crate::util::{eval_source, report_error};
 #[cfg(feature = "plugin")]
 use log::info;
+#[cfg(feature = "plugin")]
+use nu_parser::ParseError;
+#[cfg(feature = "plugin")]
+use nu_path::canonicalize_with;
 use nu_protocol::engine::{EngineState, Stack, StateWorkingSet};
+#[cfg(feature = "plugin")]
+use nu_protocol::Spanned;
 use nu_protocol::{HistoryFileFormat, PipelineData, Span};
 use std::path::PathBuf;
 
@@ -15,12 +21,13 @@ const HISTORY_FILE_SQLITE: &str = "history.sqlite3";
 pub fn read_plugin_file(
     engine_state: &mut EngineState,
     stack: &mut Stack,
+    plugin_file: Option<Spanned<String>>,
     storage_path: &str,
     is_perf_true: bool,
 ) {
     // Reading signatures from signature file
     // The plugin.nu file stores the parsed signature collected from each registered plugin
-    add_plugin_file(engine_state, storage_path);
+    add_plugin_file(engine_state, plugin_file, storage_path);
 
     let plugin_path = engine_state.plugin_signatures.clone();
     if let Some(plugin_path) = plugin_path {
@@ -43,8 +50,23 @@ pub fn read_plugin_file(
 }
 
 #[cfg(feature = "plugin")]
-pub fn add_plugin_file(engine_state: &mut EngineState, storage_path: &str) {
-    if let Some(mut plugin_path) = nu_path::config_dir() {
+pub fn add_plugin_file(
+    engine_state: &mut EngineState,
+    plugin_file: Option<Spanned<String>>,
+    storage_path: &str,
+) {
+    if let Some(plugin_file) = plugin_file {
+        let working_set = StateWorkingSet::new(engine_state);
+        let cwd = working_set.get_cwd();
+
+        match canonicalize_with(&plugin_file.item, cwd) {
+            Ok(path) => engine_state.plugin_signatures = Some(path),
+            Err(_) => {
+                let e = ParseError::FileNotFound(plugin_file.item, plugin_file.span);
+                report_error(&working_set, &e);
+            }
+        }
+    } else if let Some(mut plugin_path) = nu_path::config_dir() {
         // Path to store plugins signatures
         plugin_path.push(storage_path);
         plugin_path.push(PLUGIN_FILE);