[proc macros]: add alias attribute

[niklasad1]

We want the proc macros to support aliases for a certain method or subscription such as:

pub trait Rpc {
	/// The server should respond to both `getKeys` and `getKeysWithAlias` with some method registered on the server.
	#[method(name = "getKeys", alias = "getKeysWithAlias")]
	async fn get_keys(&self) -> Vec<usize>
}

[niklasad1]

Fixed by #442