♻️ refactor(core): rename labels module to gmail_client

- rename labels module to gmail_client for better clarity and context

src/lib.rs

@@ -9,7 +9,7 @@ mod credential;
 mod delete;
 mod eol_action;
 mod error;
-mod labels;
+mod gmail_client;
 mod message_list;
 mod processor;
 mod retention;
@@ -22,7 +22,7 @@ pub use credential::Credential;
 pub use delete::Delete;
 pub use eol_action::EolAction;
 pub use error::Error;
-pub use labels::Labels;
+pub use gmail_client::GmailClient;
 pub use message_list::DEFAULT_MAX_RESULTS;
 pub use message_list::MessageList;
 pub use processor::Processor;