Add settings source for AWS Secrets Manager

[rishacha]

What does this PR do?

This commit adds a SettingsSource for AWS Secrets Manager

Implements #175

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (e529615) 97.45% compared to head (5b37e2e) 96.58%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #176      +/-   ##
==========================================
- Coverage   97.45%   96.58%   -0.87%     
==========================================
  Files           5        5              
  Lines         314      322       +8     
  Branches       69       70       +1     
==========================================
+ Hits          306      311       +5     
- Misses          6        9       +3     
  Partials        2        2              

Files	Coverage Δ
pydantic_settings/sources.py	96.66% <62.50%> (-1.05%)	⬇️

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