Mike grogan/add coverage endpoint [Merge with caution]

Proposed Changes for helium network coverage widget grant

Adds coverage endpoint to mappers
- Finds the h3 hex according to given coordinates in params
- Returns coverage data based on real mapped data or an estimate based on any mapped hexes within a search area
- Estimations return an average of the rssi and snr values found in the surrounding area based on the algorithm below.

Query an expanding search area for any h3 hexes in mappers, discarding already checked hexes
If nothing found by 128 hex distance, stop search, return not covered
If mapped data found, stop search
Take the longest measured good signal in the area from the search
If you are closer to an uplink than that good signal, you are covered, return average values in the area.
If you are not closer, deduct the free space path loss from the rssi on the Δ of the distance, if that is still > -120 rssi, you are covered.