Fix to issue #3295 related to Anthropic bedrock

[makkzone]

Why are these changes needed?

This change will allow session token to be included in the request to anthropic via amazon bedrock.

##Related PR number

#3210

Related issue number

#3295

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation 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.