App-specific records

[jefflau]

WIP.

Things that need to be thought about:

• [ ] What is the best way to generate unique keys?
• [ ] What's the best use case for this?
• [ ] What's the best way to discover app-specific keys - or does it not matter if only the apps use it?

[varunsrin]

Overall this takes the same approach that Farcaster took with the KeyRegistry but with a few differences:

1. No validation of keys
2. No attribution to key requestor
3. No explicit double opt-in

I think 1 & 2 are fine and a general purpose system like ENS probably shouldn't have these specifics that may not apply to all apps. But i think 3 is worth considering as part of the spec