[Core] [Frontend] Priority scheduling for embeddings and in the OpenAI-API

[schoennenbeck]

This makes the new priority scheduling available to users of the OpenAI-API (and fixes some missing priority-handling for embedding-related methods).

• Added priority field to CompletionRequest, ChatCompletionRequest, and EmbeddingRequest with description
• Added priority to the generate- and encode-method of the engine protocol
• OpenAIServingChat, OpenAIServingCompletion, and OpenAIServingEmbedding handle the priority from the request in their corresponding create_*-methods.
• Added priority handling to MQLLMEngineClient.
• Added priority handling to embedding related methods in AsyncLLMEngine where previously missing

FIX #8282

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[DarkLight1337]

LGTM now, thanks!