slashing: handle equivocations between BLS sig and precommit

[SebastianElvis]

Validators with native stake have two roles: they sign precommit votes for CometBFT (using Ed25519 signatures) and also sign BLS signatures for BTC timestamps. Currently, a validator can be slashed if it signs precommit votes for two conflicting blocks. But for slashable safety, a validator should also be slashed if it signs a precommit vote for one block and a timestamp for a conflicting block.

We will need to implement a msg handler for handling such equivocation evidence that contains a precommit and a BLS sig conflicting with each other.