Validators sign this structure that allows the TwoChainTimeoutCertificate to store a round number
instead of a quorum cert per validator in the signatures field.
Cryptographic hasher for an BCS-serializable #item
This structure contains all the information necessary to construct a signature
on the equivalent of a DiemBFT v4 timeout message.
TimeoutCertificate is a proof that 2f+1 participants in epoch i
have voted in round r and we can now move to round r+1. DiemBFT v4 requires signature to sign on
the TimeoutSigningRepr and carry the TimeoutWithHighestQC with highest quorum cert among 2f+1.