[Refactor] Using the MsgEthereumTx interface in the rpc module

I understand that cosmos/evm will continue to follow up on compatibility with ethereum.
As the structure of MsgEthereumTx was modified to introduce EIP7702, the above case may occur again in the future.
I think that if we define MsgEthereumTx, which is used in the rpc module like the ante module's GaxTx, as an interface, we can consider future expandability.

Although we're not to preserve ethermint's backwards compatibility, this approach could solve that problem as well.
The above ethermint related content is related to the hardcoded `*evmtypes.MsgEthereumTx` that was discussed in Slack.