AggLayer: combine `faucet_registry_map` and `faucet_metadata_map`

> Splitting into separate maps would just multiply storage slots without buying anything.
>
> Agreed! I wonder if it would make sense to push this even further and combine `faucet_registry_map` with `faucet_metadata_map`. We could use the fourth element in sub-key `1` to track the `is_native` flag).
>
> Another question (mostly to clarify my understanding): we store the metadata hash in the map - but where does the actual metadata get stored? And why do we need 8 elements for the hash? Is this because we need to hash it using Keccak for some reason?

_Originally posted by @bobbinth in [#2700](https://github.com/0xMiden/protocol/issues/2700#issuecomment-4270543362)_