feat(pubsub): Rename Client to BasePublisher

[suzmue]

This is the first step in simplifying the Publisher client creation and renames the Client and associated builders.

For #4282

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.78%. Comparing base (b03561b) to head (6d55155).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4283      +/-   ##
==========================================
- Coverage   94.80%   94.78%   -0.02%     
==========================================
  Files         187      187              
  Lines        7194     7194              
==========================================
- Hits         6820     6819       -1     
- Misses        374      375       +1     

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[coryan]

SGTM. I think we can bikeshed the names once we are closer to "not preview".