[Access] Update signature field for CollectionGuarantee and Block

[synzhu]

For CollectionGuarantee, signatures is defined as array, https://bit.ly/2TjAQ5m, while only a single one is actually populated: https://bit.ly/3wbjrZZ
Same thing for Block:
https://bit.ly/3xeJel2
https://bit.ly/3ha2uuB

The signatures currently being used are aggregated signatures, which are intended to replace a list of multiple individual signatures.

We should update the protobuf entities to reflect the Go types.

In addition, we should also add all the other fields specified by a block, like ParentVoterIDs and ProposerID and ProposerSig (and the Payload stuff).

Since this may involve removing the old fields, it may be a breaking change for clients.

[github-actions]

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