Add support for Comet-style server side filtering

[lpsinger]

See https://comet.transientskp.org/en/stable/filtering.html.

[codecov]

Codecov Report

Attention: Patch coverage is 53.84615% with 6 lines in your changes missing coverage. Please review.

Project coverage is 71.69%. Comparing base (e453216) to head (9c501a9).

Files with missing lines	Patch %	Lines
gcn/voeventclient.py	54.54%	5 Missing ⚠️
gcn/cmdline.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #48      +/-   ##
==========================================
- Coverage   72.76%   71.69%   -1.07%     
==========================================
  Files           5        5              
  Lines         257      265       +8     
==========================================
+ Hits          187      190       +3     
- Misses         70       75       +5     

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