✨ feat(rules_cli): implement add command for managing retention rules

- add `add` subcommand to manage retention rules
- introduce `Period` enum for specifying time units
- implement logic to add and save new retention rules to the configuration

src/cli/config_cli/rules_cli/add_cli.rs

@@ -0,0 +1,50 @@
+use std::fmt;
+
+use clap::{Parser, ValueEnum};
+use cull_gmail::{Config, Error, MessageAge, Retention};
+
+#[derive(Debug, Clone, Parser, ValueEnum)]
+pub enum Period {
+    Days,
+    Weeks,
+    Months,
+    Years,
+}
+
+impl fmt::Display for Period {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Period::Days => write!(f, "days"),
+            Period::Weeks => write!(f, "weeks"),
+            Period::Months => write!(f, "months"),
+            Period::Years => write!(f, "years"),
+        }
+    }
+}
+
+#[derive(Debug, Parser)]
+pub struct AddCli {
+    /// Period for the rule
+    #[arg(short, long)]
+    period: Period,
+    /// Count of the period
+    #[arg(short, long, default_value = "1")]
+    count: usize,
+    /// Optional specific label; if not specified one will be generated
+    #[arg(short, long)]
+    label: Option<String>,
+    /// Immediate delete instead of move to trash
+    #[arg(long)]
+    delete: bool,
+}
+
+impl AddCli {
+    pub fn run(&self, mut config: Config) -> Result<(), Error> {
+        let generate = self.label.is_none();
+        let message_age = MessageAge::new(self.period.to_string().as_str(), self.count);
+        let retention = Retention::new(message_age, generate);
+
+        config.add_rule(retention, self.label.as_ref(), self.delete);
+        config.save()
+    }
+}