Add support for MAVLink Signing

This is needed if you want to make sure nobody can talk to your vehicle without a private key.

More about that [here](https://mavlink.io/en/guide/message_signing.html).

Start with the linked MAVLink message-signing guide and inspect the repository's existing MAVLink handling; the issue does not name any files or tests. Identify where signing configuration and message verification belong, then define tests for private-key authentication and rejection of unauthenticated vehicle communication.

Written by the indexing model from the issue text.