fix(rest): cache credentials

[scotthart]

fixes #9607

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This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 41279e81409840a81884dcc20b9eec0621f92d38

• Public Access Build Logs
• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.

[coryan]

Would be nice to have a test, but LGTM without it.

[codecov]

Codecov Report

Merging #9620 (41279e8) into main (e8cc9a9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #9620   +/-   ##
=======================================
  Coverage   94.35%   94.35%           
=======================================
  Files        1488     1488           
  Lines      137709   137711    +2     
=======================================
+ Hits       129936   129942    +6     
+ Misses       7773     7769    -4     

Impacted Files	Coverage Δ
google/cloud/internal/curl_rest_client.h	100.00% <ø> (ø)
google/cloud/internal/curl_rest_client.cc	96.89% <100.00%> (+0.04%)	⬆️
google/cloud/storage/parallel_upload.cc	98.26% <0.00%> (-0.35%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	98.49% <0.00%> (ø)
google/cloud/internal/curl_impl.cc	89.95% <0.00%> (+0.22%)	⬆️
...le/cloud/internal/default_completion_queue_impl.cc	97.15% <0.00%> (+0.56%)	⬆️
...loud/bigtable/internal/connection_refresh_state.cc	98.61% <0.00%> (+2.77%)	⬆️
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	100.00% <0.00%> (+2.94%)	⬆️

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