core: frontend: NMEA Injector should support specifying a GPS ID

[ES-Alexander]

Current behaviour

The message supports the field, but we're currently not specifying it, which may make filtering of separate GPSs harder for receivers (although in principle MAVLink GPSs should also be separated by component ID, which we do allow specifying).

Raised in this forum thread.

Expected or desired behaviour

The NMEA Injector service should be updated to include a GPS ID field for each configured NMEA socket.

That may also be usable to automatically set the relevant GPS_TYPEn parameter in the autopilot to MAVLink, which could resolve/help #2516.

Prerequisites

• [X] I have checked to make sure that a similar request has not already been filed or fixed.