Pretty straightforward, the `UpdateFarcasterMessageData` variant includes `creatorFID` and `targetFID`.
Considered something like
```lan=js
+userIDsToFID: { +[userID: string]: string },
```
for consistency with `FarcasterRelationshipRequest`, but think this approach is cleaner and we don't need validators/etc to ensure that there are only two entries in `userIDsToFID`, etc.
---
Depends on D11624