Group: add an agent key to use to reveal keys to parent groups and writeOnly keys to members

[gdorsi]

There are some edge cases that are not handled by the current writeOnly key system (see this test for reference) and we currently can't let accounts extend groups if they don't have access to their readKey

To fix these issues we want to add an asymmetric key on Group to make it possible to do key revelations without having access to the readKey.

This way we can:

• Do a single key revelation for the entire group when creating a writeOnly key
• Use the parent public key to reveal the current group readKey to be able to do group extension without read access (would fix #1331)

Started to draft the idea here: https://github.com/garden-co/jazz/commit/f1e9d9f6f8e088eebe04fbd0b169ae0c16c2731a

[gdorsi]

Should also fix this: https://github.com/garden-co/jazz/blob/d63716a8272502af7d35396e008af2d761c6f6da/packages/cojson/src/coValues/group.ts#L609