Add basic chat plugin harness (no plugin loading implemented)

rc_plugins/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "oj_rc_plugins"
+version.workspace = true
+edition.workspace = true
+repository.workspace = true
+license.workspace = true
+authors.workspace = true
+readme.workspace = true
+
+[dependencies]
+log.workspace = true