AggLayer: Normalize storage slot naming

[mmagician]

Maybe not for this PR, but we should normalize storage slot naming. For example, this should be miden::agglayer::bridge::let to be consistent with miden::agglayer::bridge::ger_upper and miden::agglayer::bridge::ger_lower (which I think should be just one slot miden::agglayer::bridge::ger).

Originally posted by @bobbinth in #2334 (comment)

[mmagician]

Some other notes:
miden::agglayer::faucet stores the ID of the AggLayer bridge - the slot name doesn't correspond to the actual usage.