[proc macros]: generated unsubscribe methods with subscribe in method name doesn't work.

[niklasad1]

#[rpc(client)]
pub trait RpcApi {
       #[subscription(
		name = "author_submitAndWatchExtrinsic" => "author_extrinsicUpdate",
		item = TransactionStatus<Hash, Hash>,
	)]
	fn watch_extrinsic(&self, bytes: &Bytes) -> RpcResult<()>;
}

This will generate -> "unsubscribeauthor_submitAndWatchExtrinsic", see https://github.com/paritytech/jsonrpsee/blob/master/proc-macros/src/rpc_macro.rs#L342-#L344

For the servers that shouldn't matter much but for the clients it's a big concern i.e, the unsubscribe won't work properly.
So maybe it could make sense return Option<String> instead and require users to provide unsubscribe if not the method starts with subscribe

[niklasad1]

Closed by #702