Problem: missing signer parser for MsgEthereumTx

[vincentysc]

The Signer of MsgEthereumTx should be the from address. However, a valid MsgEthereumTx should have an empty from address in Tx Messages, possibly deprecated in LegacyTx (Ref: https://github.com/evmos/ethermint/blob/main/x/evm/types/msg.go#L364).

The code shows that ValidateBasic function in Ethermint is not empty and it could be a good starting point (Ref: https://github.com/evmos/ethermint/blob/main/x/evm/types/msg.go#L165).