✨ feat(label): implement add label subcommand

- add `add` subcommand to the label cli
- allow users to add labels to existing rules

src/config_cli/label_cli/add_cli.rs

@@ -0,0 +1,19 @@
+use clap::Parser;
+
+use cull_gmail::{Config, Result};
+
+#[derive(Debug, Parser)]
+pub struct AddCli {
+    /// Id of the rule on which action applies
+    #[clap(short, long)]
+    id: usize,
+    /// Label to add to the rule
+    #[clap(short, long)]
+    label: String,
+}
+
+impl AddCli {
+    pub fn run(&self, _config: Config) -> Result<()> {
+        Ok(())
+    }
+}