Remove non-AsXx surface area from M.E.AI.OpenAI/AzureAIInference

[stephentoub]

We expect the AsChatClient/AsEmbeddingGenerator extension methods from M.E.AI.OpenAI and M.E.AI.AzureAIInference to move into the OpenAI / Azure.AI.Inference libraries, respectively. To prepare for that, and the temporary M.E.AI.OpenAI/AzureAIInference libs then being deprecated, this PR removes the other surface area from these assemblies. For anything folks found useful, we should find another way to ship it, likely just as sample source somewhere.

Closes #6094
Closes #5965
Closes #5964

Microsoft Reviewers: Open in CodeFlow

[SteveSandersonMS]

This is certainly all good stuff in general.

Within the GetService implementations, should we also remove the clauses that return this if serviceType.IsInstanceOfType(this)? I'm not sure what use case those clauses would serve now you can't just reference the type directly and can only get it in obscure ways (e.g., by being the second-to-last middleware and passing innerClient.GetType().

[stephentoub]

This should be ready to go.

One important note:
I've made two additional changes:

• Removed the AsChatClient/AsEmbeddingGenerator extensions for the OpenAIClient type, leaving just the ones for OpenAI.Chat.ChatClient and OpenAI.Embeddings.EmbeddingClient. Given what we've seen with OpenAI's move to push folks away from ChatClient and towards ResponseClient, it seems unwise for us to have AsChatClient on OpenAIClient that's bound to the chat completion implementation.
• Renamed AsChatClient to AsIChatClient and AsEmbeddingGenerator to AsIEmbeddingGenerator. This is to avoid confusion with naming, e.g. having an OpenAI ChatClient have an AsChatClient extension method on it.

cc: @jeffhandley, @SteveSandersonMS