Allow passing of a Specification object to AbstractApp.add_api

[bobbyrullo]

Description

Right now you can pass either a file path to a JSON swagger spec, or a parsed dictionary of one.

Expected behaviour

It would be nice if you could pass in a Specification object, since they can be quite expensive to create even from a dictionary - as much as 10s for ours! Enabling this could speed up startup times for large apps, as well as aid in testing where apps are created over and over.

Actual behaviour

It doesn't do allow this :) People instead monkeypatch (eg. https://github.com/zalando/connexion/issues/306#issuecomment-584258868) to work around this which is never ideal.