[Plugin] Forward RTK RTCM packets to FCU

[AlexisTM]

Adding a plugin which can send the RTCM updates of the base station to the UAVs. Thus replacing the need for QGroundcontrol for in-flight operation.

[julianoes]

Sounds like a good thing to have. Thanks for listing this.

[mrpollo]

For context, there's some information on a duplicate issue #534

[coderkalyan]

The most time consuming thing to develop here is probably a parser for GPS messages I think?

[coderkalyan]

Or... not, https://github.com/PX4/PX4-GPSDrivers this exists apparently

[AlexisTM]

The rtk_ros actually uses -an outdated version of- PX4/GPSDrivers. We were using Mavros for that RTCM forward feature.

[Katawann]

And this issue #1252 is pointing how QGC is handle it:

This is the functionality in QGC.
https://github.com/mavlink/qgroundcontrol/blob/master/src/GPS/RTCM/RTCMMavlink.cc#L24

It is sending GPS_RTCM_DATA.
https://mavlink.io/en/messages/common.html#GPS_RTCM_DATA