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/handle_custom_value.rs

@@ -0,0 +1,42 @@
+use nu_protocol::{CustomValue, LabeledError, ShellError, Span, Value};
+use serde::{Deserialize, Serialize};
+
+/// References a stored handle within the plugin
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct HandleCustomValue(pub u64);
+
+impl HandleCustomValue {
+    pub fn into_value(self, span: Span) -> Value {
+        Value::custom(Box::new(self), span)
+    }
+}
+
+#[typetag::serde]
+impl CustomValue for HandleCustomValue {
+    fn clone_value(&self, span: Span) -> Value {
+        self.clone().into_value(span)
+    }
+
+    fn type_name(&self) -> String {
+        "HandleCustomValue".into()
+    }
+
+    fn to_base_value(&self, span: Span) -> Result<Value, ShellError> {
+        Err(LabeledError::new("Unsupported operation")
+            .with_label("can't call to_base_value() directly on this", span)
+            .with_help("HandleCustomValue uses custom_value_to_base_value() on the plugin instead")
+            .into())
+    }
+
+    fn notify_plugin_on_drop(&self) -> bool {
+        true
+    }
+
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
+
+    fn as_mut_any(&mut self) -> &mut dyn std::any::Any {
+        self
+    }
+}