Support OpenAPI Extensions

[twosigmajab]

Hey Flask-Rebar friends! Have you considered adding support for OpenAPI Extensions?

At the API World conference a few months ago, we learned that folks working at Azure have come up with several extensions they're using, e.g. to advertise that an endpoint conforms to their standard way of paging results. Folks at Oracle use extensions for things like API ownership info, internal issue trackers, etc. We're interested in using extensions to customize behavior of clients generated from our specs.

OpenAPI Extensions seem potentially very useful for Flask-Rebar to be able to include in our generated specs, and it seems it wouldn't be too hard to extend the current API to accept them. What do you think?