Use scoped addresses as recipients

[nventuro]

#9380 describes a note tag as

let tag: Field = poseidon2([app_siloed_secret, recipient_address, index]);

and dismisses recipient_address as "being simply the user's address". However, this turns out to be a naive view of the problem.

We typically want to discover notes at a point in which we don't really know the user's address, e.g. get_notes and view_notes, or when PXE calls into a contract. When reading notes from e.g. a set inside a map the caller does know who they belong to, but a) a note does not necessarily belong to the account it was encrypted for, and b) this information is lost in the call chain and aztec-nr does not see this value (it is typically consumed by Map::at).

The proposed solution is to use all of PXE's scoped accounts as potential recipients, and find notes for any of them. This is a very good set: get_notes will never return notes for any account not in this set, as they are not in scope, and any new note for any of these accounts could presumably be read in the current transaction.