Add DescribeSecret cache

[simonmarty]

Issue #, if available: Closes #20

Description of changes:

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 23.97661% with 130 lines in your changes missing coverage. Please review.

Project coverage is 66.59%. Comparing base (994fe63) to head (5f1a161).

Files	Patch %	Lines
...nager_caching/src/output/describe_secret_output.rs	0.00%	89 Missing ⚠️
...nager_caching/src/secret_store/memory_store/mod.rs	4.54%	21 Missing ⚠️
aws_secretsmanager_caching/src/lib.rs	60.78%	20 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
- Coverage   69.03%   66.59%   -2.44%     
==========================================
  Files          13       14       +1     
  Lines        2842     2967     +125     
  Branches       10        9       -1     
==========================================
+ Hits         1962     1976      +14     
- Misses        880      991     +111     

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