Simplify Message Containers

[GalRogozinski]

Description

For spec it makes better sense to have simpler message containers with inefficient logic, rather than hard to decipher nested map structure.

[GalRogozinski]

See:

ssv-spec/ssv/partial_sig_container.go

Lines 14 to 19 in ccf408d

	type PartialSigContainer struct {
	// Signature map: validator index -> signing root -> operator id (signer) -> signature (from the signer for the validator's signing root)
	Signatures map[phase0.ValidatorIndex]map[SigningRoot]map[types.OperatorID]types.Signature
	// Quorum is the number of min signatures needed for quorum
	Quorum uint64
	}

[alan-ssvlabs]

Looking at this code for a while, I think although it it not good looking, it is a reasonable implementation. If we want to get rid of the nested map we can use a list of PartialSignatureMessage, but it makes the GetSignature and GetSignatures function less elegant. What is the solution in your mind?

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
}

[GalRogozinski]

@alan-ssvlabs
I think this is good actually
The implementation will be something like this (no IDE was use so may have errors):

func GetSignature(ps PartialSigContainer, validatorIndex phase0.ValidatorIndex, signer types.OperatorID, signingRoot [32]byte) (types.Signature, error) {
	for _, psm := range ps.GetSignatureMessages {
              if psm.ValidatorIndex == validatorIndex && psm.Signer == signer && psm.Root == signingRoot {
                    return psm.Signature, nil
              }
        }
        return nil, error.new("Signature is not found")
}

I think this is simpler and more readable.
But before you continue with this let's hear what @moshe-blox , @y0sher and the sigP guys think

[AgeManning]

I spoke with the team. We don't have strong opinions either way.

We typically like simpler things that are easy to read, so this change sounds good to us.