Support OpenAPI spec generation [ENHANCEMENT]

[pandu-k]

Is your feature request related to a problem? Please describe.
In Marqo 2.x there isn't a way to generate OpenAPI spec. This was possible from the /openapi.json path.

Describe the solution you'd like
An endpoint, usable from a running Marqo container, from which to retrieve the openapi specs.

Additional context
Relevant docs: https://fastapi.tiangolo.com/reference/openapi/docs/