Validate model/adapter is available on the model server before sending requests to a model server

[liu-cong]

According to Model Server Protocol, supported model servers should expose AvailableModels. In vLLM, the /v1/models endpoint exposes such info.

The ext proc should get the AvailableModels similar as scraping metrics, and add a filter to only pick model servers that have the targetModel in the request as available.