impl(pubsub): subscriber client (without rpcs)

[dbolduc]

Part of the work for #3941

Add the beginnings of a Subscriber client and its ClientBuilder. At the moment, this thing is useless, but eventually it will gain a streaming_pull method returning the StreamingPull builder.

The examples are all tagged as no_rust because none of the types involved are public yet.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.74%. Comparing base (1830c77) to head (1794b09).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4138      +/-   ##
==========================================
+ Coverage   94.72%   94.74%   +0.02%     
==========================================
  Files         184      186       +2     
  Lines        6933     6949      +16     
==========================================
+ Hits         6567     6584      +17     
+ Misses        366      365       -1     

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