♻️ refactor(cli): restructure cli commands for better organization

- rename `label_cli.rs` to `labels_cli.rs`
- rename `message_cli.rs` to `messages_cli.rs`
- move config related commands to `rules config` subcommand
- introduce `rules run` subcommand

src/cli/labels_cli.rs

@@ -0,0 +1,12 @@
+use clap::Parser;
+use cull_gmail::{Error, GmailClient};
+
+#[derive(Debug, Parser)]
+pub struct LabelsCli {}
+
+impl LabelsCli {
+    pub async fn run(&self, client: GmailClient) -> Result<(), Error> {
+        client.show_label();
+        Ok(())
+    }
+}