Enable openapi() and custom_openapi functions in model to expose OpenAPI docs

[axsaucedo]

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As a followup of #331 we should explore allowing users to use the openapi() and custom_openapi() functions that FastAPI exposes https://fastapi.tiangolo.com/advanced/extending-openapi/#normal-fastapi to allow for a simple and robust way of exposing OpenAPI docs. We would have to take into consideration how this could be supported for the multi-model server use-case, namely on whether each path would have its high level management API swagger doc together with the per-model swagger API, or a top level OpenAPI that contains all the information. The latter seems more preferrable as that could allow for simpler discoverability of endpoints available.