rpc-v2/transaction: Generate `Invalid` events and add tests (#3784)

This PR ensures that the transaction API generates an `Invalid` events
for transaction bytes that fail to decode.

The spec mentioned the `Invalid` event at the jsonrpc error section,
however this spec PR makes things clearer:
- https://github.com/paritytech/json-rpc-interface-spec/pull/146

While at it have discovered an inconsistency with the generated events.
The drop event from the transaction pool was incorrectly mapped to the
`invalid` event.

Added tests for the API stabilize the API soon:
- https://github.com/paritytech/json-rpc-interface-spec/pull/144


Closes: https://github.com/paritytech/polkadot-sdk/issues/3083


cc @paritytech/subxt-team

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