eth_coinbase address does not correspond to an account

[yihuang]

System info: [Include Ethermint commit, operating system name, and other relevant details]

Steps to reproduce:

Currently, what coinbase api returns is the consensus address, there's no correspond bank account for it, and I believe it's not the "address that staking rewards will be send to" as mentioned in comment. We should do a conversion(GetValidatorByConsAddr) and return the validator address to avoid confusion.
But we don't have direct access to the keeper in eth rpc layer, we need to add a convert api in cosmos-sdk first.

[fedekunze]

correct, technically speaking, the coinbase address is the address that mining rewards will be send to. We can return the validator address in hex format by adding support to this staking query on the evm Query Service