In PartialSignatureMessages we can move signing roots to header

[GalRogozinski]

Description

We have:

// PartialSignatureMessage is a msg for partial Beacon chain related signatures (like partial attestation, block, randao sigs)
type PartialSignatureMessage struct {
	PartialSignature Signature `ssz-size:"96"` // The Beacon chain partial Signature for a duty
	SigningRoot      [32]byte  `ssz-size:"32"` // the root signed in PartialSignature
	Signer           OperatorID
	ValidatorIndex   phase0.ValidatorIndex
}

type PartialSignatureMessages struct {
	Type PartialSigMsgType
	Slot phase0.Slot
	Messages []*PartialSignatureMessage `ssz-max:"5048"`
}

The signingRoot is expected to match across duties.
If we have more than one expected signing root we can use indexes.

[github-actions]

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