Allow bridged assets from Asset Hub

[alistair-singh]

What does it do?

Allows Asset Hub to be a reserve for bridged assets (parents > 2). Allows Asset Hub to be a reserve for DOT.

What important points reviewers should know?

Allows Asset Hub to be a reserve for DOT.

This may be an issue supporting DOT from both the relaychain and Asset Hub. We need to support our Oneclick transfers feature. We can remove this, but then users would need to bridge to Asset Hub and then move the asset from Asset Hub to Moonbeam in a separate step. More info in this forum post: https://forum.polkadot.network/t/managing-sas-on-multiple-reserve-chains-for-same-asset/7538

Important: Does the Moonbeam runtime allow for execution to be paid in DOT?

Is there something left for follow-up PRs?

We still need to bed down UX .e.g How do users get assets from Asset Hub to Moonbeam and back. This may require follow up PRs.

What alternative implementations were considered?

Are there relevant PRs or issues in other repositories (Substrate, Polkadot, Frontier, Cumulus)?

What value does it bring to the blockchain users?

Bridging in liquidity from Kusama and Ethereum to Polkadot. Allowing these assets to be used on Dapps on Moonbeam.