Accept CashAddr prefixes in bitcoin-tx input

bitcoin-tx accepts CashAddr addresses as input, but errors if the address includes an explicit prefix (i.e. bitcoincash:, bchtest:, or bchreg:). This fixes the error.

This deals with bitcoin-tx input, while !274 (merged) deals with bitcoin-tx output. Merge either this or !274 (merged), and I'll rebase the other one.

Test plan: ninja check (this includes the bitcoin-tx tests)

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