Snowbridge: Support bridging native ETH (#6855)

Changes:
1. Use the 0x0000000000000000000000000000000000000000 token address as
Native ETH.
2. Convert it to/from `{ parents: 2, interior:
X1(GlobalConsensus(Ethereum{chain_id: 1})) }` when encountered.

Onchain changes:
This will require a governance request to register native ETH (with the
above location) in the foreign assets pallet and make it sufficient.

Related solidity changes:
https://github.com/Snowfork/snowbridge/pull/1354

TODO:
- [x] Emulated Tests

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Co-authored-by: Vincent Geddes <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>