[Rococo<>Westend bridge] Allow any asset over the lane between the two Asset Hubs (#4888)

On Westend Asset Hub, we allow Rococo Asset Hub to act as reserve for
any asset native to the Rococo or Ethereum ecosystems (practically
providing Westend access to Ethereum assets through double bridging:
W<>R<>Eth).

On Rococo Asset Hub, we allow Westend Asset Hub to act as reserve for
any asset native to the Westend ecosystem. We also allow Ethereum
contracts to act as reserves for the foreign assets identified by the
same respective contracts locations.

- [x] add emulated tests for various assets (native, trust-based,
foreign/bridged) going AHR -> AHW,
- [x] add equivalent tests for the other direction AHW -> AHR.

This PR is a prerequisite to doing the same for Polkadot<>Kusama bridge.