.NET: Improve fidelity of OpenAI ChatCompletions Hosting (#1785)
* rename, support json serialization
* wip
* non-streaming
* streaming?
* proper streaming types
* comments + fix audio parse
* copilot suggestions
* proper stopsequences type
* build options as i could
* annotations
* proper generation of Id for chatcompletions
* string length as in chatcompletions api ref
* image url
* support tools
* rework API
* introduce tests for chatcompletions
* function calling / serialization tests / fixes
* more tests and coverage
* fix format
* sort usings
* nit
* address PR comments
* nits
Korolev Dmitry