MAST: count_only parameter for MastMissions queries

[snbianco]

Quick PR to add a count_only parameter to MastMissions query methods. If this parameter is set, the method will return a count of results rather than the full result table. Also updated docs and tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.43%. Comparing base (b0eb232) to head (cbf2570).
⚠️ Report is 20 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3645   +/-   ##
=======================================
  Coverage   73.42%   73.43%           
=======================================
  Files         230      230           
  Lines       21362    21368    +6     
=======================================
+ Hits        15686    15692    +6     
  Misses       5676     5676           

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