Add --root-path to specificy HTTP prefix

[alvarobartt]

What does this PR do?

This PR adds the --root-path argument so that users can specify the prefix for the HTTP API, similarly as in vLLM with FastAPI, so that instead of having http://localhost:3000/v1/embeddings, one might set --root-path jinaai/jina-embeddings-v2-base-en meaning the HTTP routes are behind http://localhost:3000/jinaai/jina-embeddings-v2-base-en instead so that the OpenAI Embeddings API sits in http://localhost:3000/jinaai/jina-embeddings-v2-base-en/v1/embeddings.

Closes #888

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