[Gateway] Misc fixes to Message parsing, signing and validation

[bolekk]

1. Set Message.Sender as a non-serializable field.
2. Add Message.Receiver field. Because messageID is scoped to a user, nodes need to set receiver to that user’s address to make sure responses can be uniquely attributed to users and also to prove to the user that it is in fact a response to their message.
3. Fix ValidateSignature(). I mistakenly assumed that previous logic was enough to fully validate a signature but we always need to validate the expected sender. Renamed the function to ExtractSigner() to avoid any confusion.
4. Perform message validation before calling handler methods, as suggested by @pinebit. Even though there are theoretical use cases that might not need it, I don’t think they are going to be implemented in the near future.
5. Changing HandleGatewayMessage() attribute back to full message instead of just body. This will be needed for a very real use case, where nodes want to share received messages among each other, within the OCR round. It’s also consistent with the Gateway Handler interface.
6. Fix broken scripts.

[github-actions]

I see that you haven't updated any README files. Would it make sense to do so?

[cl-sonarqube-production]

SonarQube Quality Gate

71.0% Coverage on New Code (is less than 80%)

See analysis details on SonarQube

[KuphJr]

LGTM