Implement transaction signing on `FuelWalletConnector`

[nedsalk]

As reported on the forum, the user is trying to sign a transaction but is hitting a not-implemented method via Account.signTransaction when doing this call.

UPDATE:

To address this issue, we need to take the following steps:

1 - Add the signTransaction method to the FuelConnectorMethods enum. This ensures that the Fuel connector manager will correctly initialize any connector implementing this method during the execution of the setupMethods function.

2 - Implement the signTransaction method in FuelWalletConnector.