[omm] Interfaces for storage

[Dcallies]

Summary

Adds a shim for storage for a few interfaces, including a mock for storage.

TODO: I know I need to add threatexchange to a dependency somewhere, but I didn't see it (looked for a setup.py), will either add it in a followup PR once I find it or add it here. I suspect I just need to read CONTRIBUTING.

Test Plan

For now, just python repl

>>> import OpenMediaMatch.storage.default as s
>>> inst = s.DefaultOMMStore()
>>> inst.get_signal_type_configs()
{'pdq': SignalTypeConfig(enabled=True, signal_type=<class 'threatexchange.signal_type.pdq.signal.PdqSignal'>), 'video_md5': SignalTypeConfig(enabled=True, signal_type=<class 'threatexchange.signal_type.md5.VideoMD5Signal'>)}

[Dcallies]

Discussed offline with @dougneal - currently hard to see how things connect, so as a fast follow will try and add the interfaces for bank (name, enabled), and then add a mocked version, with the intent to add a post-gres backed impl when I get back from PTO.