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_custom_values/src/generate2.rs

@@ -66,6 +66,6 @@ impl SimplePluginCommand for Generate2 {
 fn test_examples() -> Result<(), nu_protocol::ShellError> {
     use nu_plugin_test_support::PluginTest;
 
-    PluginTest::new("custom_values", crate::CustomValuePlugin.into())?
+    PluginTest::new("custom_values", crate::CustomValuePlugin::new().into())?
         .test_command_examples(&Generate2)
 }