Serve cache on transient errors

[benjaminkz]

STSGetCallerIdentity, DescribeSecret and GetSecretValue requests may fail because of common network errors like Sdkerror::Timeout and server-side errors like Sdkerror::ServiceError<Box, HttpResponse>. This cr adds a new configurable parameter ignore_transient_errors. With that enabled, the agent will return the cached secret when running into common transient errors like the above.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 84.10256% with 31 lines in your changes missing coverage. Please review.

Project coverage is 70.59%. Comparing base (c604b4b) to head (db3e1af).

Files with missing lines	Patch %	Lines
aws_secretsmanager_caching/src/lib.rs	84.39%	27 Missing ⚠️
aws_secretsmanager_agent/src/config.rs	80.00%	2 Missing ⚠️
aws_secretsmanager_caching/src/error.rs	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #34      +/-   ##
==========================================
+ Coverage   68.80%   70.59%   +1.79%     
==========================================
  Files          14       15       +1     
  Lines        2885     2850      -35     
  Branches       10        9       -1     
==========================================
+ Hits         1985     2012      +27     
+ Misses        900      838      -62     

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