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

This PR backports the changes from
https://github.com/paritytech/json-rpc-interface-spec/pull/107

.

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

// cc @paritytech/subxt-team

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Signed-off-by: Alexandru Vasile <[email protected]>