[Whitelisted Caller] Make WETH asset sufficient and set asset metadata

Since Snowbridge has launched and the ERC-20 Wrapped Ether token (WETH) has been registered on AssetHub, set WETH to be a sufficient asset (allow the WETH account to exist without requiring an existential deposit in DOT or another sufficient asset). This ensures a better UX for new users who may not have a DOT balance on AssetHub, even though the Snowbridge UI will warn the user before attempting a transfer without the required existential deposit of a sufficient asset.

Additionally, this referendum sets WETH's metadata to match the metadata on Ethereum:

{ name: 'Wrapped Ether', symbol: 'WETH', decimals: 18 }