MP need to connect to Pixhawk via 2 communication Links

#### Issue details
Communication Link between MP and Pixhawk sometimes get broken. In order to mitigate the broken. we can use 2 communication links for the communication between MP and pixhawk, and pixhawk has been support 2 communication links, but MP does not. MP could support 2 links, one is main, another is supplementary and MP can switch between these 2 links to select the one which can reach pixhawk.

#### Version
_What version was the issue encountered with_

#### Platform
[X] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Sub

#### Airframe type
_What type of airframe (flying wing, glider, hex, Y6, octa etc)_
For all.
#### Hardware type
_What autopilot hardware was used? (pixhawk, pixracer, PX4FMU etc)_
For all
#### Logs
_Please provide a link to any relevant logs that show the issue_

No files, tests, or entry points are named. Start by tracing how Mission Planner currently manages its communication link with Pixhawk, then compare that flow with Pixhawk's support for two links. Done means Mission Planner can configure a main and supplementary link and switch between them to reach Pixhawk when one is broken.

Written by the indexing model from the issue text.