impl(pubsub): StreamingPull builder

[dbolduc]

Part of the work for #3941

Add a builder for a streaming pull bidi RPC. This class will eventually be public, and thus it must have documentation.

For now, this thing just sets options. It will gain a fn start(self) -> Result<Session> later, when Session is a thing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.82%. Comparing base (ddd9cb9) to head (639d81b).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4118      +/-   ##
==========================================
- Coverage   94.84%   94.82%   -0.03%     
==========================================
  Files         181      182       +1     
  Lines        6853     6864      +11     
==========================================
+ Hits         6500     6509       +9     
- Misses        353      355       +2     

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[dbolduc]

PTAL