Currently evidence is only added to the evidence pool from the consensus layer, as this is typically where evidence is first detected. However evidence can also potentially be detected by light-clients and we need to provide a way for them to submit that to nodes. We should add a new /broadcast_evidence endpoint (open to better naming) that takes all the relevant information - height, round, validator address, blockID, and two conflicting signatures.

As part of this, we should also add a tool for generating conflicting signatures to simplify testing. cc cosmos/cosmos-sdk#2111