MAVLink 2 signed-frame trailer.
MAVLink 2 signing appends a 13-byte trailer after the checksum: one-byte link id, 48-bit timestamp, and 48-bit signature.
@type t() :: %XMAVLink.Frame.Signature{ link_id: 0..255, signature: <<_::48>>, timestamp: 0..281_474_976_710_655 }