[WIP] Set GPS to stationary mode when fixed position is enabled

[fifieldt]

untested WIP - this code may never run, need to check where GPS setup bails out due to fixed_position config.

As reported by @RCGV1, some GPS chips have a "stationary" mode, which assumes acceleration and velocity are zero. For fixed nodes, taking advantage of this configuration means potentially less power usage, and a more accurate position that doesn't jitter its way around the map. (See also: https://github.com/meshtastic/firmware/issues/3984 )

This patch makes changes to the setup code for the the GNSS_MODEL_MTK, GNSS_MODEL_MTK_L76B, GNSS_MODEL_ATGM336H, GNSS_MODEL_AG3335, GNSS_MODEL_AG3352 to apply that approach.

The aim would be to enable this mode when users have a GPS enabled and select "Fixed Position" without specifying coordinates. UX TBD!

Fixes https://github.com/meshtastic/firmware/issues/6627

🤝 Attestations

• [ ] I have tested that my proposed changes behave as described.
• [ ] I have tested that my proposed changes do not cause any obvious regressions on the following devices:
  • [ ] Heltec (Lora32) V3
  • [ ] LilyGo T-Deck
  • [ ] LilyGo T-Beam
  • [ ] RAK WisBlock 4631
  • [ ] Seeed Studio T-1000E tracker card
  • [ ] Other (please specify below)

[garthvh]

Fixed position does not generally use the gps at all, I would suggest the stationary mode should not need any UX at all and instead not use power or generate positions all over the place when the gps detects it is stationary. For this to be done well it should reduce the positions sent over the mesh, save power and not require any more position settings as there are already far too many.