Add support for image generation endpoint

[stuartleeks]

See https://learn.microsoft.com/en-us/azure/ai-services/openai/reference#image-generation

• add generator for image generation and include in default generators
• validate image generation endpoint with record/replay
• determine how rate-limiting is applied and add to the simulator rate-limiting
• add tests
• add load test

(from stuartleeks/aoai-simulated-api#48)

[martinpeck]

Questions:

1. how many variations of images do we need?
2. I assume that we don't need to show a "relevant" image. Correct?
3. There are two ways images can be returned: as a URL or as Base64. Does the URL need to resolve to anything?
4. If the url needs to resolve, do we need to create all of the sizes, image types etc? I don't see any need (I can't imagine that this will impact perf testing or other load testing.

[stuartleeks]

1. how many variations of images do we need?
   I guess we can start with one for the initial implementation and add more later?

2. I assume that we don't need to show a "relevant" image. Correct?
   Correct - if we're faking the image then it doesn't need to be relevant.

3. There are two ways images can be returned: as a URL or as Base64. Does the URL need to resolve to anything?
   I think it would be good for the URL to resolve to an endpoint in the API that will serve up the image content. That way any client calling the API and getting the URL can still follow the URL to get the content

4. If the url needs to resolve, do we need to create all of the sizes, image types etc? I don't see any need (I can't imagine that this will impact perf testing or other load testing.
   As with image variation, I suggest we start simple and expand later

[stuartleeks]

For the record/replay mode, I imagine that the base64 image option should just work. For the url response the recorder will need to do some work to change the returned URL in the response so that it resolves via the simulator (and can then record the request to the image URL as well)

[martinpeck]

I'm going to un-assign myself from this.