Make STIXFrame work nicely with sunpy Maps

[samaloney]

Often want/need to do analysis in the STIX frame (really low counts) so need to support repojection and plotting etc. This PR aims to facilitate the creation of maps in the STIXImaging frame and support all the plotting, reprojections etc. Heavily based on what in sunpy coordinates.

• Register new frame and wcs mapping to astropy
• Update STIXImage to include observer and rsun

[samaloney]

Well after some serious rabbit holing, mostly on the plotting side finally made some progress. Still lots to fix, improve (transforms are slow due to repeated fido search and fetch calls) and look into (I don't know why the limb is behind, indicated by the dotted line, the Sun).

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.40260% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 66.33%. Comparing base (73f8d02) to head (079da8c).

Files	Patch %	Lines
stixpy/coordinates/frames.py	96.42%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #90 +/- ## ========================================== + Coverage 64.25% 66.33% +2.07% ========================================== Files 29 31 +2 Lines 1757 1821 +64 ========================================== + Hits 1129 1208 +79 + Misses 628 613 -15 ```

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[samaloney]

Current plot