fix(common): censor Bearer tokens over HTTP/2

[coryan]

Our logs censored Bearer tokens over HTTP/1.x. These tokens appear in the "info" callbacks, not the "header" callbacks for HTTP/2, and we failed to censor the tokens in that case.

This change is 

[scotthart]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @coryan)

[codecov]

Codecov Report

Attention: Patch coverage is 95.65217% with 1 line in your changes missing coverage. Please review.

Project coverage is 93.59%. Comparing base (424ca3c) to head (5d132a4).
Report is 97 commits behind head on main.

Files with missing lines	Patch %	Lines
google/cloud/internal/curl_wrappers_test.cc	83.33%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #14564   +/-   ##
=======================================
  Coverage   93.58%   93.59%           
=======================================
  Files        2318     2318           
  Lines      206995   207000    +5     
=======================================
+ Hits       193723   193733   +10     
+ Misses      13272    13267    -5     

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