add enabled extension and implement overlay plugin

Signed-off-by: Schmarni <marnistromer@gmail.com>

crates/bevy_openxr/src/openxr/exts.rs

@@ -1,6 +1,9 @@
-use bevy::prelude::{Deref, DerefMut};
+use bevy::{ecs::system::Resource, prelude::{Deref, DerefMut}};
 use openxr::ExtensionSet;
 
+#[derive(Clone, Debug, Eq, PartialEq, Deref, DerefMut, Resource)]
+pub struct OxrEnabledExtensions(pub OxrExtensions);
+
 #[derive(Clone, Debug, Eq, PartialEq, Deref, DerefMut)]
 pub struct OxrExtensions(ExtensionSet);
 impl OxrExtensions {