Add CLI variable gps_dyn_model allowed value AIR_2G - for improved accuracy

[MrMap]

Current Behavior

Current choices of GPS dynamic model does not include the middle/ "normal" option that would be AIR_2G.
Allowed values today: PEDESTRIAN, AIR_1G, AIR_4G Default AIR_1G

Desired Behavior

AIR_2G to be included in allowed values - since uBlox supports it.
Might benefit user/dev tuning of position estimator engine to increase accuracy / reduce lag.

Suggested Solution

Add GPS dynamic model AIR_2G as a valid option.

Who does this impact? Who is this for?

It´s for anyone attempting to tune the position estimator to balance accuracy / lag.
Current choices might represent too cold or too hot. Maybe AIR_2g could represent sweet spot.

Additional context

Copy paste from UBLOX:

Airborne <1g Used for applications with a higher dynamic range and greater vertical
acceleration than a passenger car. No 2D position fixes supported.
Airborne <2g Recommended for typical airborne environments. No 2D position fixes
supported.
Airborne <4g Only recommended for extremely dynamic environments. No 2D position fixes
supported.

UBLOX data sheet:
https://www.u-blox.com/sites/default/files/products/documents/u-blox8-M8_ReceiverDescrProtSpec_%28UBX-13003221%29.pdf