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-protocol/src/value/custom_value.rs

@@ -27,6 +27,9 @@ pub trait CustomValue: fmt::Debug + Send + Sync {
     /// Any representation used to downcast object to its original type
     fn as_any(&self) -> &dyn std::any::Any;
 
+    /// Any representation used to downcast object to its original type (mutable reference)
+    fn as_mut_any(&mut self) -> &mut dyn std::any::Any;
+
     /// Follow cell path by numeric index (e.g. rows)
     fn follow_path_int(
         &self,