Add reply chat completion client

[MohMaz]

Why are these changes needed?

• Adds a ReplayChatCompletionClient client so we can mimic LLM API calls locally with predefined set of responses with some native implementation of usage logic

Related issue number

Closes #3622

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.

[ekzhu]

Looks like a great start! Can we move this to autogen-ext package under autogen_ext.models so we can use it across tests and examples in all packages.