Allow implicit aws credential setting for AnthropicBedrockChatCompletionClient

[GeorgeEfstathiadis]

Why are these changes needed?

Allows implicit AWS credential setting when using AnthropicBedrockChatCompletionClient in an instance where you have already logged into AWS with SSO and credentials are set as environment variables.

Related issue number

Closes #6560

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.52%. Comparing base (bd3b97a) to head (f2721b7).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6561   +/-   ##
=======================================
  Coverage   79.52%   79.52%           
=======================================
  Files         225      225           
  Lines       16624    16628    +4     
=======================================
+ Hits        13220    13224    +4     
  Misses       3404     3404           

Flag	Coverage Δ
unittests	79.52% <100.00%> (+<0.01%)	⬆️

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