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-test-support/src/plugin_test.rs

@@ -136,7 +136,7 @@ impl PluginTest {
             move |mut value| match PluginCustomValue::serialize_custom_values_in(&mut value) {
                 Ok(()) => {
                     // Make sure to mark them with the source so they pass correctly, too.
-                    PluginCustomValue::add_source(&mut value, &source);
+                    let _ = PluginCustomValue::add_source_in(&mut value, &source);
                     value
                 }
                 Err(err) => Value::error(err, value.span()),

crates/nu-plugin-test-support/tests/custom_value/mod.rs

@@ -35,6 +35,10 @@ impl CustomValue for CustomU32 {
         self
     }
 
+    fn as_mut_any(&mut self) -> &mut dyn std::any::Any {
+        self
+    }
+
     fn partial_cmp(&self, other: &Value) -> Option<Ordering> {
         other
             .as_custom_value()