Serve JSON spec too

[zupo]

Besides serving the YAML spec, we should serve the JSON spec too.

See #9 (review).

[sayanarijit]

Should we make add_spec_view auto-detect the file extension and serve the spec in that format?

e.g.

This will serve YAML spec:

add_spec_view(
    filepath: "/openapi.yaml",
    route: str = "/openapi.yaml",
)

This will serve JSON spec:

add_spec_view(
    filepath: "/openapi.yaml",
    route: str = "/openapi.json",
)

Also, I'm inclining towards defaulting route to /openapi.json regardless of the filepath format. Because that spec will mostly be read by programs/non-human entities. For humans, there is a UI based explorer.