feat: SDSS-V (astra) model spectra default loader

[rileythai]

This PR addresses #1183 , and adds a default loader for the model spectra from SDSS-V's astra with relevant tests.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 87.06%. Comparing base (dd1d60b) to head (21a5baa).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1203      +/-   ##
==========================================
+ Coverage   86.90%   87.06%   +0.15%     
==========================================
  Files          63       63              
  Lines        4576     4645      +69     
==========================================
+ Hits         3977     4044      +67     
- Misses        599      601       +2     

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[rileythai]

i forgot to mark this as ready 2 months ago, should be good now

[rosteen]

@rileythai I rebased this and am seeing test failures - would you double check against your local branch and make sure I didn't botch something in the rebase? This was originally from before the 2.0 release, so there certainly could be changes needed to get it up to date beyond what I did to resolve the conflicts. Apologies for missing this when you initially marked it ready for review!