.NET: implement own OpenAI type-model and remove OpenAI SDK as a dependency

[DeagleGross]

Both OpenAI Responses and ChatCompletions implementation in agent-framework rely on the official OpenAI SDK NuGet package, which has a "non-traditional" type model definition. That includes:

1. no public exposure for some of openai protocol types (e.g. StreamingResponseOutputItemAddedUpdate). One could not instantiate such a type.
2. some of members (like ResponseCreationOptions.Stream) are not public.

Workaround today is to rely on the reflection, but that will easily break the consumers. So we should move from using OpenAI SDK sooner rather than later.