Support for getting help text from a plugin command (#12243)

# Description
There wasn't really a good way to implement a command group style (e.g.
`from`, `query`, etc.) command in the past that just returns the help
text even if `--help` is not passed. This adds a new engine call that
just does that.

This is actually something I ran into before when developing the dbus
plugin, so it's nice to fix it.

# User-Facing Changes


# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting
- [ ] Document `GetHelp` engine call in proto

crates/nu-plugin/src/protocol/mod.rs

@@ -447,6 +447,8 @@ pub enum EngineCall<D> {
     GetCurrentDir,
     /// Set an environment variable in the caller's scope
     AddEnvVar(String, Value),
+    /// Get help for the current command
+    GetHelp,
     /// Evaluate a closure with stream input/output
     EvalClosure {
         /// The closure to call.
@@ -474,6 +476,7 @@ impl<D> EngineCall<D> {
             EngineCall::GetEnvVars => "GetEnvs",
             EngineCall::GetCurrentDir => "GetCurrentDir",
             EngineCall::AddEnvVar(..) => "AddEnvVar",
+            EngineCall::GetHelp => "GetHelp",
             EngineCall::EvalClosure { .. } => "EvalClosure",
         }
     }
@@ -491,6 +494,7 @@ impl<D> EngineCall<D> {
             EngineCall::GetEnvVars => EngineCall::GetEnvVars,
             EngineCall::GetCurrentDir => EngineCall::GetCurrentDir,
             EngineCall::AddEnvVar(name, value) => EngineCall::AddEnvVar(name, value),
+            EngineCall::GetHelp => EngineCall::GetHelp,
             EngineCall::EvalClosure {
                 closure,
                 positional,