Allow implicit aws credential setting for AnthropicBedrockChatCompletionClient (#6561)

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## 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.

Co-authored-by: Jack Gerrits <jackgerrits@users.noreply.github.com>