Make drop notification timing for plugin custom values more consistent (#12341)

# Description
This keeps plugin custom values that have requested drop notification
around during the lifetime of a plugin call / stream by sending them to
a channel that gets persisted during the lifetime of the call.

Before this change, it was very likely that the drop notification would
be sent before the plugin ever had a chance to handle the value it
received.

Tests have been added to make sure this works - see the `custom_values`
plugin.

cc @ayax79 

# User-Facing Changes
This is basically just a bugfix, just a slightly big one.

However, I did add an `as_mut_any()` function for custom values, to
avoid having to clone them. This is a breaking change.

crates/nu-plugin/src/lib.rs

@@ -67,7 +67,6 @@ mod plugin;
 mod protocol;
 mod sequence;
 mod serializers;
-mod util;
 
 pub use plugin::{
     serve_plugin, EngineInterface, Plugin, PluginCommand, PluginEncoder, PluginRead, PluginWrite,
@@ -88,6 +87,8 @@ pub use plugin::{
 pub use protocol::{PluginCustomValue, PluginInput, PluginOutput};
 #[doc(hidden)]
 pub use serializers::EncodingType;
+#[doc(hidden)]
+pub mod util;
 
 // Used by external benchmarks.
 #[doc(hidden)]