Create auto add unknown features adapter

[jnunemaker]

Original discussion here:
#271 (comment)

I wouldn't just check features on every get as that adds extra queries. But we could memoize known features and check against that in memory set. Then refresh the set of known features every minute or two or if an unknown feature is seen. That would likely avoid the extra flipper calls due to features that are in code but never added or were deleted.

[travisbell]

Hi @jnunemaker, this will be a great addition! Thanks for seeing the value and agreeing to get it done. 🙏🏼