save collapsed spectrum to FITS

[cshanahan1]

This PR adds functionality to the cubeviz collapse plugin to save an output FITS file.

[kecnry]

Hopefully this won't stay like this for long and we can prioritize a more general solution soon, but until then this looks good - thanks!

[codecov]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (bd312bd) 91.51% compared to head (6e1c863) 91.52%.

Files	Patch %	Lines
...plugins/spectral_extraction/spectral_extraction.py	86.48%	5 Missing ⚠️
...daviz/configs/default/plugins/collapse/collapse.py	86.48%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #2586    +/-   ##
========================================
  Coverage   91.51%   91.52%            
========================================
  Files         161      161            
  Lines       19596    19724   +128     
========================================
+ Hits        17934    18052   +118     
- Misses       1662     1672    +10     

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[javerbukh]

Tested for a while and works well! If you can show a snackbar message when the file already exists and the user presses "save as fits", this will be ready for merge.

[cshanahan1]

i added the overwrite overlay to the save button if the file already exists