skycoord obstime and equinox support

[calgray]

When using differentials with SkyCoords, obstime is an important data property for the calculating the coordinate at other epochs such as when using apply_space_motion(new_obstime=new_time). For frames that don't support obstime, SkyCoord has a transitive optional obstime constructor argument.

There is likewise also an optional equinox argument for SkyCoord that's not as useful, but can also be supported for completeness.

• Closes #xxxx
• Tests added
• Passes pre-commit run --all-files
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.12%. Comparing base (ab9964b) to head (7c14ecb).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
+ Coverage   85.93%   86.12%   +0.18%     
==========================================
  Files          14       14              
  Lines        1152     1153       +1     
  Branches      179      178       -1     
==========================================
+ Hits          990      993       +3     
+ Misses        115      114       -1     
+ Partials       47       46       -1     

Flag	Coverage Δ
unittests	86.03% <100.00%> (+0.18%)	⬆️

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Files with missing lines	Coverage Δ
astropy_xarray/coordinates/frame.py	90.43% <100.00%> (+1.54%)	⬆️
astropy_xarray/coordinates/sky_coord.py	97.91% <100.00%> (+0.13%)	⬆️

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