Add versioned signatures for Transparent transactions

[ConstanceBeguier]

Add SighashInfo (version and associated data) to

• Transparent authorizing signatures.

Update auth_digest by adding SighashInfo as defined in ZIP246

Add SighashInfo for Sapling/Orchard binding signatures only when the signatures exist

[PaulLaux]

Approved with minor comments

[PaulLaux]

replace x with vin

[ConstanceBeguier]

Done

[PaulLaux]

Later, in

        for desc in tx.vOutputsSapling:
            digest.update(bytes(desc.proof))
        digest.update(write_compact_size(len(tx.bindingSigSaplingInfo)))
        digest.update(bytes(tx.bindingSigSaplingInfo))
        digest.update(bytes(tx.bindingSigSapling))

what happens if tx.bindingSigSaplingInfo in None?

[ConstanceBeguier]

def sapling_auth_digest_v6(tx):
    digest = blake2b(digest_size=32, person=SAPLING_AUTH_DIGEST_PERSONALIZAION)

    if len(tx.vSpendsSapling) + len(tx.vOutputsSapling) > 0:
        for desc in tx.vSpendsSapling:
            digest.update(bytes(desc.proof))
        for desc in tx.vSpendsSapling:
            digest.update(write_compact_size(len(desc.spendAuthSigInfo)))
            digest.update(bytes(desc.spendAuthSigInfo))
            digest.update(bytes(desc.spendAuthSig))
        for desc in tx.vOutputsSapling:
            digest.update(bytes(desc.proof))
        digest.update(write_compact_size(len(tx.bindingSigSaplingInfo)))
        digest.update(bytes(tx.bindingSigSaplingInfo))
        digest.update(bytes(tx.bindingSigSapling))

    return digest.digest()

If tx.bindingSigSaplingInfo is None and we try to access it, an exception will be raised.
The function sapling_auth_digest_v6 is only called for v6 transaction.
In this function, tx.bindingSigSaplingInfo and tx.bindingSigSapling are accessed only when the transaction includes a Sapling element.
For v6 transactions, whenever a Sapling element is present, both of these fields are populated.