[WIP] Add diffusers serve to CLI

[DN6]

What does this PR do?

• Introduce diffusers-cli serve to serve a diffusers compatible model/pipeline based on the GMS spec.
• Introduce diffusers-cli generate client to make it easy to send GMS compatible requests to the server.
• Support SGLang and vLLM-omni backends

TODO:

• Common serving optimisations related to concurrency are still not implemented.

Fixes # (issue)

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