This is the initial specification of how session keys can be registered on AH as well.

Note that for Kusama deployment, we will go with the old flow where session key mgmt is 100% on the relay chain. It will just require a configurable amount of deposit. See #1625

For Polkadot, we have two options:

1. We keep the session pallet on RC, but the #[pallet::calls] are filtered/removed. Validators would submit their session keys to AH, put deposit down, in StakingRcClient, and they are immediately sent to RC. In other words, the origin of Session::set_keys should be EnsureAssetHub. This is a Noop for existing validators. The staking election would still report stash keys to the RC, and RC's session pallet would map them to the right session key.
2. Validators would register their session keys on AH, and they are kept there. AH would send session keys directly yo AH upon completion of staking election.

For now I think option 1 is more reasonable + path of least resistance.