[Azure OpenAI] Image generation support from 2023-06-01-preview service API

[trrwilson]

This change introduces DALL-E image generation support to the .NET SDK based on the latest TypeSpec code generation updates.

The code is a bit "thick" with custom code relative to what we'd like, but that's necessary at the moment given that Azure OpenAI performs image generation as a long-running operation (LRO) while standard OpenAI performs it as a normal, synchronous POST operation.

To represent this,

• Response<ImageGenerations> GetImageGenerationsAsync(ImageGenerationOptions, ...) is introduced, parallel in structure to e.g. Response<ChatCompletions> GetChatCompletionsAsync(..., ChatCompletionsOptions, ...)
• ImageGenerations contains a collection of ImageResponseItem elements, each of them a concrete instance of one of the two derived classes: ImageLocation and ImagePayload
• Which of the two concrete types populates the abstract collection of IReadOnlyList<ImageResponseItem> Data is determined by the request value of ResponseFormat in the ImageGenerationOptions
• For consuming format-specific data (URL or base64 string), consumers need to cast the item to the appropriate derived type, e.g. (imageGenerationsResponse.Value.Data[0] as ImageLocation).Url
• The abstract class also provides a generalized Stream GetStream() method that will generate a stream for the image data independently of the response format (using the client's pipeline to facilitate downloads in the case of URL format and simply wrapping the base64 data in the case of base64 format)

Internally, the implementation of GetImageGenerations wraps a deferred LRO call in the case of Azure OpenAI requests and constructs a simpler, straightforward request in the case of standard OpenAI service use.

These capabilities are based on the 2023-06-01-preview service API and we update that dependency (including test collateral) as part of this.

[azure-sdk]

API change check

APIView has identified API level changes in this PR and created following API reviews.

Azure.AI.OpenAI

[trrwilson]

Subsumed by the converged PR: #37539