feat: Conversion of Anthropic Extension to M.E.AI interface [WIP]

[lokitoth]

Why are these changes needed?

Enables use of multiple backends (as in 0.2) with 0.4 using the Microsoft.Extensions.AI interfaces.

Related issue number

Progress for #3919

Remaining to be done

• [x] AutoGen.Anthropic
• [ ] AutoGen.Gemini
• [ ] AutoGen.Mistral
• [ ] AutoGen.WebAPI
• [ ] AutoGen.DotnetInteractive (depends on AgentChat PR, coming soon)

Unclear if still needed with the M.E.AI.AIFunction

• [ ] AutoGen.SourceGenerator?

These got done via MEAI hosting in Microsoft.AutoGen.Extensions.MEAI

• [x] AutoGen.Ollama
• [x] AutoGen.OpenAI
• [x] AutoGen.AzureAIInference

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.

[LittleLittleCloud]

I feel like this PR can benefit much more .NET developers if moving to M.E.AI library, what you think

[lokitoth]

I feel like this PR can benefit much more .NET developers if moving to M.E.AI library, what you think

Agreed; the point of Microsoft.Autogen.Extensions.Anthropic is to provide an IChatClient implementation of the Anthropic APIs until there is an official variant. The only reason we keep the old infrastructure is that I did not want to rebuild everything by hand and have two copies.