feat: make ctrlc available to plugins (#13181)

# Description

This PR adds a new method to `EngineInterface`: `register_ctrlc_handler`
which takes a closure to run when the plugin's driving engine receives a
ctrlc-signal. It also adds a mirror of the `signals` attribute from the
main shell `EngineState`.

This is an example of how a plugin which makes a long poll http request
can end the request on ctrlc:
https://github.com/cablehead/nu_plugin_http/blob/main/src/commands/request.rs#L68-L77

To facilitate the feature, a new attribute has been added to
`EngineState`: `ctrlc_handlers`. This is a Vec of closures that will be
run when the engine's process receives a ctrlc signal.

When plugins are added to an `engine_state` during a `merge_delta`, the
engine passes the ctrlc_handlers to the plugin's
`.configure_ctrlc_handler` method, which gives the plugin a chance to
register a handler that sends a ctrlc packet through the
`PluginInterface`, if an instance of the plugin is currently running.

On the plugin side: `EngineInterface` also has a ctrlc_handlers Vec of
closures. Plugin calls can use `register_ctrlc_handler` to register a
closure that will be called in the plugin process when the
PluginInput::Ctrlc command is received.

For future reference these are some alternate places that were
investigated for tying the ctrlc trigger to transmitting a Ctrlc packet
through the `PluginInterface`:

- Directly from `src/signals.rs`: the handler there would need a
reference to the Vec<Arc<RegisteredPlugins>>, which would require us to
wrap the plugins in a Mutex, which we don't want to do.

- have `PersistentPlugin.get_plugin` pass down the engine's
CtrlcHandlers to .get and then to .spawn (if the plugin isn't already
running). Once we have CtrlcHandlers in spawn, we can register a handler
to write directly to PluginInterface. We don't want to double down on
passing engine_state to spawn this way though, as it's unpredictable
because it would depend on whether the plugin has already been spawned
or not.

- pass `ctrlc_handlers` to PersistentPlugin::new so it can store it on
itself so it's available to spawn.

- in `PersistentPlugin.spawn`, create a handler that sends to a clone of
the GC event loop's tx. this has the same issues with regards to how to
get CtrlcHandlers to the spawn method, and is more complicated than a
handler that writes directly to PluginInterface

# User-Facing Changes

No breaking changes

---------

Co-authored-by: Ian Manske <ian.manske@pm.me>

crates/nu-plugin-engine/src/interface/mod.rs

@@ -1,7 +1,7 @@
 //! Interface used by the engine to communicate with the plugin.
 
 use nu_plugin_core::{
-    util::{with_custom_values_in, Sequence, Waitable, WaitableMut},
+    util::{with_custom_values_in, Waitable, WaitableMut},
     Interface, InterfaceManager, PipelineDataWriter, PluginRead, PluginWrite, StreamManager,
     StreamManagerHandle,
 };
@@ -11,8 +11,8 @@ use nu_plugin_protocol::{
     PluginOutput, ProtocolInfo, StreamId, StreamMessage,
 };
 use nu_protocol::{
-    ast::Operator, CustomValue, IntoSpanned, PipelineData, PluginMetadata, PluginSignature,
-    ShellError, Signals, Span, Spanned, Value,
+    ast::Operator, engine::Sequence, CustomValue, IntoSpanned, PipelineData, PluginMetadata,
+    PluginSignature, ShellError, Signals, Span, Spanned, Value,
 };
 use nu_utils::SharedCow;
 use std::{
@@ -664,6 +664,12 @@ impl PluginInterface {
         self.flush()
     }
 
+    /// Send the plugin a ctrl-c signal.
+    pub fn ctrlc(&self) -> Result<(), ShellError> {
+        self.write(PluginInput::Ctrlc)?;
+        self.flush()
+    }
+
     /// Write an [`EngineCallResponse`]. Writes the full stream contained in any [`PipelineData`]
     /// before returning.
     pub fn write_engine_call_response(