[AHs] Support registering assets on Asset Hubs over bridge (#5435)

## Changes

Allows one Asset Hub on one side, to register assets on the other Asset
Hub over the bridge.

Rococo <> Ethereum test bridge will be dropped in favor of Westend <>
Ethereum test bridge. This PR also changes emulated tests to simulate
double bridging from Ethereum<>Westend<>Rococo.

## Tests

- [x] e2e test: Westend Asset Hub can register Westend and Ethereum
assets on Asset Hub Rococo
- [x] e2e test: Rococo Asset Hub can register Rococo assets on Asset Hub
Westend
- [x] e2e test (existing): Users on Ethereum can register Ethereum
assets on Asset Hub Westend
- [x] e2e test: transfer Rococo assets from Rococo to Westend and back
- [x] e2e test: transfer Westend assets from Westend to Rococo and back
- [x] e2e test: transfer Ethereum assets (bridged in through Snowbridge)
from Westend to Rococo and back