Actually store outgoing notes

[benesjan]

In this PR I've implemented all the plumbing necessary to process the outgoing notes. The missing piece there is actually storing the note in the PXE db. I didn't do this because we are currently unable to recover note hash index for the outgoing because for that we would need a compute_note_hash function (the compute_note_hash_and_nullifier(...) function reverts when we can't get a hold of nullifier keys).

Look for "TODO(#6867)" in the codebase to find where the missing functionality should be implemented.