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.

tests/plugins/custom_values.rs

@@ -181,6 +181,20 @@ fn drop_check_custom_value_prints_message_on_drop() {
     assert!(actual.status.success());
 }
 
+#[test]
+fn handle_make_then_get_success() {
+    // The drop notification must wait until the `handle get` call has finished in order for this
+    // to succeed
+    let actual = nu_with_plugins!(
+        cwd: "tests",
+        plugin: ("nu_plugin_custom_values"),
+        "42 | custom-value handle make | custom-value handle get"
+    );
+
+    assert_eq!(actual.out, "42");
+    assert!(actual.status.success());
+}
+
 #[test]
 fn custom_value_in_example_is_rendered() {
     let actual = nu_with_plugins!(