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-command/src/database/values/sqlite.rs

@@ -379,6 +379,10 @@ impl CustomValue for SQLiteDatabase {
         self
     }
 
+    fn as_mut_any(&mut self) -> &mut dyn std::any::Any {
+        self
+    }
+
     fn follow_path_int(
         &self,
         _self_span: Span,