Overhaul the plugin cache file with a new msgpack+brotli format (#12579)

# Description

- Plugin signatures are now saved to `plugin.msgpackz`, which is
brotli-compressed MessagePack.
- The file is updated incrementally, rather than writing all plugin
commands in the engine every time.
- The file always contains the result of the `Signature` call to the
plugin, even if commands were removed.
- Invalid data for a particular plugin just causes an error to be
reported, but the rest of the plugins can still be parsed

# User-Facing Changes

- The plugin file has a different filename, and it's not a nushell
script.
- The default `plugin.nu` file will be automatically migrated the first
time, but not other plugin config files.
- We don't currently provide any utilities that could help edit this
file, beyond `plugin add` and `plugin rm`
  - `from msgpackz`, `to msgpackz` could also help
- New commands: `plugin add`, `plugin rm`

# Tests + Formatting

Tests added for the format and for the invalid handling.

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting

- [ ] Check for documentation changes
- [ ] Definitely needs release notes

crates/nu-cmd-plugin/src/util.rs

@@ -0,0 +1,50 @@
+use std::fs::{self, File};
+
+use nu_engine::{command_prelude::*, current_dir};
+use nu_protocol::PluginCacheFile;
+
+pub(crate) fn modify_plugin_file(
+    engine_state: &EngineState,
+    stack: &mut Stack,
+    span: Span,
+    custom_path: Option<Spanned<String>>,
+    operate: impl FnOnce(&mut PluginCacheFile) -> Result<(), ShellError>,
+) -> Result<(), ShellError> {
+    let cwd = current_dir(engine_state, stack)?;
+
+    let plugin_cache_file_path = if let Some(ref custom_path) = custom_path {
+        nu_path::expand_path_with(&custom_path.item, cwd, true)
+    } else {
+        engine_state
+            .plugin_path
+            .clone()
+            .ok_or_else(|| ShellError::GenericError {
+                error: "Plugin cache file not set".into(),
+                msg: "pass --plugin-config explicitly here".into(),
+                span: Some(span),
+                help: Some("you may be running `nu` with --no-config-file".into()),
+                inner: vec![],
+            })?
+    };
+
+    // Try to read the plugin file if it exists
+    let mut contents = if fs::metadata(&plugin_cache_file_path).is_ok_and(|m| m.len() > 0) {
+        PluginCacheFile::read_from(
+            File::open(&plugin_cache_file_path).map_err(|err| err.into_spanned(span))?,
+            Some(span),
+        )?
+    } else {
+        PluginCacheFile::default()
+    };
+
+    // Do the operation
+    operate(&mut contents)?;
+
+    // Save the modified file on success
+    contents.write_to(
+        File::create(&plugin_cache_file_path).map_err(|err| err.into_spanned(span))?,
+        Some(span),
+    )?;
+
+    Ok(())
+}