Add AH-next runtime - the post-migration Asset Hub runtime (WIP)

[seadanda]

The migration of staking, governance, balances and other related pallets from the Relay Chain to Asset Hub is currently in development (part of the Plaza project and RFC-32).

We can already have the future state of Asset Hub ready to test against for ourselves and other stakeholders who will be integrating with it, without needing to have a completed migration. Therefore by having two asset hubs on Westend in the lead up to the transition allows us to have the old runtime and future runtime live at the same time, and crucially for the future runtime to be live far in advance of the migration being ready.

We want to the migration on Westend to be a proper dry run for Kusama and Polkadot, but want to have a testing target much before then.

TODO:

• Update from master to get blocknumber provider changes for relevant pallets
• Transplant recent changes to AH here
• Update staking pallets to the multi-block setup when the PRs are merged
• Integrate the staking interface pallet when it's ready (#rc-)

Related to https://github.com/paritytech/devops/issues/3604