fix(rest): prevent libcurl callback from reading bad address

[dbolduc]

Backport #14615 and #14617 to the v2.25.x branch, preparing for v2.25.1.

Description of changes from @scotthart:

For a Patch, Post, or Put request, curl_multi_perform should result in all the data from the associated WriteVector being consumed before it returns. Except in the case of a timeout, in which case we do not want libcurl to attempt to transmit the rest of the data while we read any remaining response data to populate the Status error message.

No need to update the ABI dumps as no public symbols have changed, and we only encode the major and minor versions in the inline namespace.

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This change is 

[scotthart]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @dbolduc)

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Please upload report for BASE (v2.25.x@aaf88be). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
google/cloud/internal/curl_impl.cc	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             v2.25.x   #14626   +/-   ##
==========================================
  Coverage           ?   93.77%           
==========================================
  Files              ?     2268           
  Lines              ?   201861           
  Branches           ?        0           
==========================================
  Hits               ?   189297           
  Misses             ?    12564           
  Partials           ?        0           

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