feat!: Add store configuration support for SSM store

[bhavanki]

The new Config and SetConfig methods on the Store interface allow
implementations to maintain their own configurations. Only the SSM store
fully implements the methods; the others return an empty configuration
and do not support setting one.

The store configuration itself holds a list of required tags for each
written secret. Enforcement of the tags is not yet implemented. Although
the configuration struct is exported from its package, it is not part of
chamber's client interface and is subject to change at any time.

The SSM store implementation stores its configuration as a secret
inside the newly reserved "_chamber" service as a JSON document. The
schema is not exported, so users shouldn't build anything from it.

[codecov]

Codecov Report

Attention: Patch coverage is 43.58974% with 22 lines in your changes missing coverage. Please review.

Project coverage is 36.27%. Comparing base (401c3ae) to head (a98febd).
Report is 4 commits behind head on master.

Files	Patch %	Lines
store/ssmstore.go	61.90%	4 Missing and 4 partials ⚠️
store/nullstore.go	0.00%	6 Missing ⚠️
store/s3store.go	0.00%	6 Missing ⚠️
store/secretsmanagerstore.go	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #532      +/-   ##
==========================================
+ Coverage   36.16%   36.27%   +0.11%     
==========================================
  Files          29       29              
  Lines        2552     2591      +39     
==========================================
+ Hits          923      940      +17     
- Misses       1550     1568      +18     
- Partials       79       83       +4     

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[sindhusegment]

thanks for explaining this in stand up!