Verify signatures of justifications

[dknopik]

QBFT messages that we receive from the network may have justifications attached. These justifications must have valid signatures. Adjust the message_validator crate to verify the signatures of all attached justifications, ideally in the existing validate_justifications function

[dknopik]

Another one that might be a good one for @petarjuki7 to get started

[diegomrsantos]

Should we label it with "good first issue"?

[dknopik]

@diegomrsantos People outside of the org tend to look for the "good first issue" tag, I think this one would be good for Petar to handle, so I didn't tag it "good first issue"