ERC20 Asset Transactor

[franciscoaguirre]

Partially addresses #7523.

In order to interact with ERC20 tokens via XCM (with DepositAsset, WithdrawAsset instructions), we need an Asset Transactor that can match them and call their contracts.

I implemented the fungibles::* family of traits for pallet-revive.

Still a work in progress.

Open questions

• What about gas? It shouldn't be charged when the impl calls the contract since we assume it has been paid for before by the XCM executor. Same when calling view functions
• Should we transfer from/to a checking account when we mint/burn fungibles?