✨ feat(cli): add label command-line interface

- implement `LabelCli` struct with `run` method for fetching Gmail labels
- integrate `clap` for command-line argument parsing

src/label_cli.rs

@@ -0,0 +1,12 @@
+use clap::Parser;
+use cull_gmail::{Error, Labels};
+
+#[derive(Debug, Parser)]
+pub struct LabelCli {}
+
+impl LabelCli {
+    pub async fn run(&self, credential_file: &str) -> Result<(), Error> {
+        let _ = Labels::new(credential_file, true).await?;
+        Ok(())
+    }
+}