Refactor ADS-B input support in Mission Planner

[MUSTARDTIGERFPV]

What this PR accomplishes

This PR refactors a significant portion of ADS-B support in Mission Planner. In the process of developing MavADSB I ran into a good number of bugs and quirks in the Mission Planner support for ADS-B input data. As I found these issues, I sought to resolve them and improve the operation of the whole component.

Changes

• [Feature] Built-in ADS-B API support - just add an HTTP base URL after enabling in Planner settings. I've spoken with @unresolv about this and she's offered to let us make https://api.adsb.lol the default; I've set it to that for now.
  • Note: This links to https://ardupilot.org/planner/docs/common-adsb.html (nonexistent) where we should add other available API endpoints)
• [Feature] ADSB_VEHICLE MAVLink uplink is limited to 1Hz, round-robin closest 10 planes within 10km. This keeps ADS-B in on the ground station from overwhelming your telemetry link. The previous approach sent known vehicles every time there was an update - enough to completely fill a telemetry link's bandwidth many times over. This approach sends only the amount necessary for up-to-date collision avoidance.
• [Feature] ADSB tooltip values follow Mission Planner preferred unit settings; unit labels are shown as well.
• [Feature] ADSB tooltip includes distance to MAV
• [Feature] ADSB tooltip includes MAV's collision threat level if enabled
• [Fix] ADSB_VEHICLES sent to the MAV, then sent back down are now ignored
• [Fix] Corrected unit scaling for altitude. Previous math had a 7% error.
• [Fix] ADSB vehicles work at all zoom levels. Previously, certain zoom levels wouldn't work if connected to a MAV with avoidance enabled due to a divide-by-0 bug with the zoom level.
• [Fix] Sped up HTTP load times; defaults to 1Hz update rate and respects HTTP 429 responses for rate limiting.
• [Fix] Resolved multi-write issues in collision threat level field
• [Minor] If MAV home location is unset, we'll pull ADS-B info over HTTP for the viewport location. Nice for checking functionality before connecting a MAV
• [Minor] Fixed unit consistency in ADS-B parsing between different input sources; all sources are now normalized to the standard Ardupilot units then transformed into display units