Add basic chat plugin harness (no plugin loading implemented)

rc_plugins/src/chat/mod.rs

@@ -0,0 +1,2 @@
+mod plugin;
+pub use plugin::{ChatProvider, ChatPlugin};

rc_plugins/src/chat/plugin.rs

@@ -0,0 +1,8 @@
+pub trait ChatPlugin: crate::Plugin {
+    fn set_provider(&self, provider: std::sync::Arc<Box<dyn ChatProvider>>);
+    fn on_message(&self, message: &str, channel: &str, username: &str);
+}
+
+pub trait ChatProvider: Send + Sync {
+    fn send_message(&self, message: &str, channel: &str, username: &str);
+}