rpc-v2/tx: Rename transaction to transactionWatch (paritytech#3040)

This PR backports the changes from
paritytech/json-rpc-interface-spec#107.

The `transaction` class becomes `transactionWatch`, and the other
functionality remains the same.

// cc @paritytech/subxt-team

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

substrate/client/rpc-spec-v2/src/transaction/api.rs

@@ -29,8 +29,8 @@ pub trait TransactionApi<Hash: Clone> {
 	/// See [`TransactionEvent`](crate::transaction::event::TransactionEvent) for details on
 	/// transaction life cycle.
 	#[subscription(
-		name = "transaction_unstable_submitAndWatch" => "transaction_unstable_watchEvent",
-		unsubscribe = "transaction_unstable_unwatch",
+		name = "transactionWatch_unstable_submitAndWatch" => "transactionWatch_unstable_watchEvent",
+		unsubscribe = "transactionWatch_unstable_unwatch",
 		item = TransactionEvent<Hash>,
 	)]
 	fn submit_and_watch(&self, bytes: Bytes);