Starting point for AOAI + OAI

[johanste]

Added starting point/skeleton for how we can include both Azure OpenAI and OpenAI endpoints in the same TSP compilation/set of files.

• I am using typespec versioning for models. And I use the feature we were going to deprecate (or at least not use for Azure Core) to facilitate the delta between OAI and AOAI. It works in limited scenarios (e.g. two targets where one is consistently a super set of the other). It does not scale (well) to more branches (e.g. OAI public, OAI internal, AOAI public, AOAI internal) - or even OAI individually versioned and AOAI individually versioned. I think it will work in this specific scenario, however.
• I made the namespace for models versioned. And have two separate services that reference a specific version from the versioned model namespace. The fact that I wanted the models namespace versioned meant I had to drag it out of the OpenAI namespace. Which is a bit unfortunate, but not at all crazy for the openapi emitter....
• I am redefining routes for the AOAI endpoints (because the routes are, in fact, different).
• I have not separated the Azure specific models into separate files.

There are still many endpoints that need to be added on the Azure side/models to be augmented. But I'd like to get feedback on the approach first.

[johanste]

I broke out the models into a separate, versioned namespace so the individual @service-decorated namespaces could reference the specific version they needed.

[johanste]

All models in Azure should be super-sets of OpenAI. And if they are not, we could accommodate that with marking them as @removed(OpenAIFlavors.Azure).

[johanste]

The outputs as provided in the PR are the result of tsp compileing azuremain.tsp.