[P<>K bridge] Allow multiple assets to be transferred between AssetHubs over bridge

[acatangiu]

To be tested first on Rococo <> Westend, then also deployed to Polkadot <> Kusama.

Main change required is the IsReserve filter on the asset hubs. New filter suggestion:

On AssetHub Polkadot: trust AHK as reserve for all assets native to Kusama:

matches!(id, { 2, GlobalConsensus(Kusama), .. })

This will allow any Kusama-based asset to move into Polkadot.

On AssetHub Kusama: trust AHP as reserve for all assets not native to Kusama:

matches!(id, { 2, GlobalConsensus(ecosystem), .. } if ecosystem != Kusama)

This will allow any non-Kusama asset to come to Kusama through Polkadot. Not just Polkadot assets, but also assets bridged to Polkadot from Ethereum/etc.

[acatangiu]

Implemented in #4888 (testnets) and polkadot-fellows/runtimes#421 (live networks).