Feat: AzureOpenAI image generations first class support

[VisargD]

Gateway now has /images/generations as a unified route which is used for providers like openai, stability, bedrock, etc. The current azure-openai provider integration does not support image generations. This should be added to allow azure-openai dall-e-3 users to make use of gateway.

Docs link: https://learn.microsoft.com/en-us/azure/ai-services/openai/dall-e-quickstart?tabs=dalle3,command-line&pivots=programming-language-studio