Support Dash AssetUnlock transactions

[huggix]

Summary

• add the missing Dash transaction types 7-9
• disable witness parsing for Dash transaction serialization/deserialization
• permit zero inputs only for AssetUnlock (type 9)
• preserve zero-input rejection for every other Dash transaction type

Why

Dash AssetUnlock transactions are valid DIP-2 special transactions with no inputs. Serializing one currently throws The transaction must have at least one input, which prevents NBXplorer from indexing past a block containing one.

Tests

• regression vector from mainnet transaction c84ef8acba47d55cff09e97ed2e29da25c954b8c40cc5c41cb73a6e1e06b5e74
• verifies decode, type, empty inputs, output value, transaction hash, and byte-for-byte round trip
• verifies an ordinary zero-input Dash transaction remains rejected
• all 10 Dash tests pass on current master (net8.0)
• the same 10 tests pass against the NBitcoin 9.0.0 / NBitcoin.Altcoins 5.0.0 source used by NBXplorer 2.5.30-1 (net6.0)

Reference: https://docs.dash.org/projects/core/en/develop/docs/reference/transactions-special-transactions.html#assetunlocktx