MAST: get_product_urls for Observations

[cpparts]

Adds a MAST Observation method (get_product_urls) that returns a list of URLs for products without downloading them. This addresses #2850. These changes have been reviewed/guided by @snbianco, and I am looking forward to feedback!

[codecov]

Codecov Report

❌ Patch coverage is 94.87179% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.27%. Comparing base (9bfbce7) to head (2532c02).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
astroquery/mast/observations.py	94.87%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3639      +/-   ##
==========================================
+ Coverage   73.24%   73.27%   +0.03%     
==========================================
  Files         230      230              
  Lines       21299    21327      +28     
==========================================
+ Hits        15601    15628      +27     
- Misses       5698     5699       +1     

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[snbianco]

Hey Celia! This looks great (I've already reviewed this PR on a different branch in my repository). To get the two failing checks to pass, you need to add an entry to the CHANGES.rst file. Put in under the "Service fixes and enhancements" subheading, and then "mast."

For the docs build, you can find the warning on line 748 of the output. I think in this case, you need to add a second layer of backticks to the text it's highlighting in the function docstring.