Snaps: Dynamic Permissions

Ideally, most or all of the Snap permissions can be requested dynamically. A snap should be able to request, say, "endowment:ethereum-provider" or "snap_notify" when it needs to enable a related feature for a user, rather than having to request it at installation. There will need to be:

- A permission request modal flow (needs design)
- A way to specify an optional permission in the manifest (via a caveat), so that the Snap has to state that it may request the permission dynamically (hopefully this does not require a manifest version change)
- A controller / API for `snap_requestPermissions` (rather than exposing `wallet_requestPermissions`)

Questions:
1. How to integrate with PermissionsController
2. How to handle requesting with wallet_requestPermissions
3. Do we allow fully dynamic caveats, partial caveats, or not at all?
4. Does this mean each permission can be revoked? Can the Snap distinguish between static and dynamic permissions when doing snap_getPermissions? Conclusion: static permissions should not be revokable. Thus, we have a concept of "deferred request for a static permission."

"We should build this such that we give the user more power but we don't lock ourselves into a pattern long term. It may be that the midterm solution is to disallow having a permission in both static and dynamic namespace, because that can get complicated really fast."

First output is a SIP

Milestones:
1. No duplicate permissions
2. Possible merging of caveats
3. Signer Snaps makes this less important