rpc: Add `z_spendoutputs` method to allow spending an explicit set of prevouts.

[nuttycom]

The API here would be relatively simple; it would completely circumvent TXO selection in favor of taking as inputs:

• an explicit set of (txid, pool, output_index) tuples to identify the TXOs to be spent
• a payment request to satisfy (or equivalent z_sendmany output specification, if we want to use that.

This would produce a PCZT for creating the requested transaction.