Self validate against known_user_agents

[be-smith]

Checks that the api is contained in KNOWN_USER_AGENTS on the datalab server. Raises a warning if not. Requires datalab-org/datalab#1806 to work

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 44.82%. Comparing base (3c1521a) to head (fc31047).

Files with missing lines	Patch %	Lines
src/datalab_api/_base.py	66.66%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #97      +/-   ##
==========================================
+ Coverage   44.55%   44.82%   +0.27%     
==========================================
  Files           4        4              
  Lines         514      522       +8     
==========================================
+ Hits          229      234       +5     
- Misses        285      288       +3     

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