remove dependency of validator.State from BitSetSignature.Verify

[tsachiherman]

Why this should be merged

This PR minimize the interface scope of the pChainState interface argument in BitSetSignature.Verify.

How this works

Existing interface of BitSetSignature.Verify is

func (s *BitSetSignature) Verify(
	ctx context.Context,
	msg *UnsignedMessage,
	networkID uint32,
	pChainState validators.State,
	pChainHeight uint64,
	quorumNum uint64,
	quorumDen uint64,
)

This PR replaces the interface with

func (s *BitSetSignature) Verify(
	msg *UnsignedMessage,
	networkID uint32,
	validators CanonicalValidatorSet,
	quorumNum uint64,
	quorumDen uint64,
) error

with the CanonicalValidatorSet defined as

type CanonicalValidatorSet struct {
	Validators  []*Validator
	TotalWeight uint64
}

The CanonicalValidatorSet construction function was added as well :

func GetCanonicalValidatorSetFromState(
        ctx context.Context,
	pChainState validators.State,
	pChainHeight uint64,
	sourceChainID ids.ID,
) (CanonicalValidatorSet, error) 

How this was tested

Existing testing were modified.

Need to be documented in RELEASES.md?

No.

Corresponding changes

coreth : ava-labs/coreth#769

[StephenButtolph]

The comment:

// Because [signers] is a subset of [vdrs], this can never error.
sigWeight, _ := SumWeight(signers)

is no longer valid right?