[ADS-B] Add emitter category support & HTTP endpoint for retrieving ADS-B data

[MUSTARDTIGERFPV]

This PR adds the following:

Differentiated icons for light/small/large/heavy and similar aircraft types. This lets users differentiate a 747 from a C172 at a glance. Thanks to tar1090 for these sprites.

Instead of retaining different sprites per color, I'm instead coloring at runtime and caching to improve performance.

Before, all the same size/shape

After, with size/shape differences matching their size/shape

Type/Category readouts in the popover to make the size & type data more explicit.

In addition to the shape/size of the sprite changing, you can also explicitly read what type of aircraft you're looking at and its category.

An endpoint on the internal webserver at /adsb which serves Mission Planner's ADS-B data as JSON

This is extremely useful for external applications to understand local traffic. Whether you're using a MAVLink-connected ADS-B In receiver or the HTTP API support I recently added, you have a unified view of the same data your MAV & MP have.

Category support on ADSB_VEHICLE messages sent to the MAV

Previously the messages sent to the MAV didn't include the category (light, small, heavy) of the aircraft. Now it does.

Speech callouts for ADS-B Traffic at emergency proximities

When traffic is very close both horizontally and vertically, turning on the speech "ADS-B Alerts" checkbox in Planner settings will give a TCAS-style callout "Traffic: 4 O'Clock high; descend" every 5 seconds.

[robertlong13]

I like it. Looks good to me. Why do you have it marked as draft? Other than the fact that you should squash that last commit into an earlier one, this looks good to go.

[MUSTARDTIGERFPV]

I like it. Looks good to me. Why do you have it marked as draft? Other than the fact that you should squash that last commit into an earlier one, this looks good to go.

I'd like to change the behavior of speech callouts by setting the thresholds a little further out and adding a 5km, 3km, and 1km warning level. Other than that, should be good to go. :)