Implement support for astronomy-1.2.0 manifest

[braingram]

This PR adds support for the tags defined in the new astronomy-1.2.0 manifest (see asdf-format/asdf-standard#455) and makes several updates to schemas in this package to favor tag instead of $ref usage to decouple the schemas.

This adds support galexsec time format.

Closes: #292

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.40%. Comparing base (add8743) to head (fb622b3).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #270   +/-   ##
=======================================
  Coverage   99.39%   99.40%           
=======================================
  Files          64       64           
  Lines        2330     2334    +4     
=======================================
+ Hits         2316     2320    +4     
  Misses         14       14           

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