impl(rust): template for an http client

[dbolduc]

Add a Rust template for http client library code, without the crate structure.

cc: @whuffman36

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.53%. Comparing base (d6fa957) to head (f59131a).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1843   +/-   ##
=======================================
  Coverage   80.53%   80.53%           
=======================================
  Files          74       74           
  Lines        8074     8074           
=======================================
  Hits         6502     6502           
  Misses       1223     1223           
  Partials      349      349           

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[coryan]

Ahh, yes. Good idea, so we can have GAPICs as part of a larger crate.