This new relationship action will set KNOW_OF for the viewer and the provided users, and create a personal thread. In a later diff, it will also add a robotext message about the Farcaster relationship.

In the future, relationship actions will get moved to the peer-to-peer Olm session, as will the personal thread creation.

Tested this in a later commit by hooking it into FarcasterDataHandler (D11387). Can't put up that code quite yet, as it depends on some components from other team members