Nick E

GCS_MAVLink: Support requesting MESSAGE_INTERVAL via REQUEST_MESSAGE cmd

2

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