GCS_MAVLink: Support requesting MESSAGE_INTERVAL via REQUEST_MESSAGE cmd

[nexton-winjeel]

Updates the MAV_CMD_REQUEST_MESSAGE handler with a special case for requesting the MESSAGE_INTERVAL message. The old method of using MAV_CMD_GET_MESSAGE_INTERVAL is deprecated.

Raised in this Discord message: https://discord.com/channels/674039678562861068/728017546313466047/1293342302261219329