Add relay identity management and subscription enhancements.

- Introduced relay identity management for subscriptions and follow-list sync.
- Added `IdentityRequested` function to handle the `identity` subcommand.
- Implemented periodic follow-list synchronization for active subscribers.
- Enhanced payment handling to include payer pubkey and subscription updates.
- Added trial expiry and subscription expiry notifications.

app/config/config.go

@@ -139,6 +139,21 @@ func GetEnv() (requested bool) {
 	return
 }
 
+// IdentityRequested checks if the first command line argument is "identity" and returns
+// whether the relay identity should be printed and the program should exit.
+//
+// Return Values
+//   - requested: true if the 'identity' subcommand was provided, false otherwise.
+func IdentityRequested() (requested bool) {
+	if len(os.Args) > 1 {
+		switch strings.ToLower(os.Args[1]) {
+		case "identity":
+			requested = true
+		}
+	}
+	return
+}
+
 // KV is a key/value pair.
 type KV struct{ Key, Value string }