Add hints into proofs to help validation

[afilini]

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[afilini]

Should we make this mandatory?

[afilini]

After a few months, an update: as suggested by @gabridome we should also add the confirmation blockheight (when known) to allow nodes without txindex to easily validate these proofs.

I would personally make the txid mandatory, while the blockheight would be optional for obvious reasons.

Also, a versioning system should be added to make it easier to deserialize proofs, to avoid the "try-catch" method where you assume some fields are present and fallback to a different strategy if they are not.

[gabridome]

@afilini or the blockhash directly.
It depends what we want to preserve: cpu or storage/bandwidth.