Previously, we used `repeated bytes` for the type of the deposit transactions in
`MsgApplyL1Txs`. Unfortunately, this led to arbitrary byte strings unmarshaling
into the `MsgApplyL1Txs` type. By encapsulating each deposit tx in a separate
`EthDepositTx` type, we avoid this issue.

In preparation for splitting the MsgApplyL1Txs type into multiple messages,
we add a Cosmos SDK transaction type that will wrap all deposits.

MsgSetL1Attributes or MsgApplyUserDeposit