Enable setting and removing fixed position via remote admin

[djholt]

Based on recent firmware change to allow remote admin position config.

Question: do we need to handle old firmware versions with this change?

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 33.33333% with 8 lines in your changes missing coverage. Please review.

Project coverage is 60.91%. Comparing base (48a06c6) to head (40353a3). Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
meshtastic/node.py	0.00%	8 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #675 +/- ## ========================================== - Coverage 60.94% 60.91% -0.04% ========================================== Files 24 24 Lines 3690 3689 -1 ========================================== - Hits 2249 2247 -2 - Misses 1441 1442 +1 ``` | [Flag](https://app.codecov.io/gh/meshtastic/python/pull/675/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=meshtastic) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/meshtastic/python/pull/675/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=meshtastic) | `60.91% <33.33%> (-0.04%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=meshtastic#carryforward-flags-in-the-pull-request-comment) to find out more.

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[ianmcorvidae]

Looks good! Thanks for picking this up, I hadn't gotten to it yet. I don't think we need to worry about older firmware versions. They'll reject any updates that get sent by this, but if someone hasn't updated they're probably aware of that constraint anyway.