Add method TreeHead.Verify
Intended to simplify StateManagerSingle.AddCosignature, see https://gitlab.glasklarteknik.se/sigsum/core/log-go/-/blob/main/internal/state/single.go#L90
Intended to simplify StateManagerSingle.AddCosignature, see https://gitlab.glasklarteknik.se/sigsum/core/log-go/-/blob/main/internal/state/single.go#L90