feat: reduce the amount of key hashing required to fetch public keys

[nventuro]

Closes https://github.com/AztecProtocol/aztec-packages/issues/7825

We now hash each set of public keys once instead of twice by having a smarter oracle function that always returns the correct value, instead of two that we need to call conditionally.

This reduces the gate count of a token transfer by ~3k, though this reduction is expected to be lower and closer to ~1k once #7954 is closed, since we'll then be performing fewer total key read operations.