script/sign: basic signing support for Miniscript descriptors

Try to solve a script using the Miniscript satisfier if the legacy
solver fails under P2WSH context. Only solve public key and public key
hash challenges for now.

We don't entirely replace the raw solver and especially rule out trying to
solve CHECKMULTISIG-based multisigs with the Miniscript satisfier since
some features, such as the transaction input combiner, rely on the
specific behaviour of the former.

test/functional/wallet_miniscript.py

@@ -59,7 +59,7 @@ class WalletMiniscriptTest(BitcoinTestFramework):
             lambda: len(self.ms_wo_wallet.listunspent(minconf=0, addresses=[addr])) == 1
         )
         utxo = self.ms_wo_wallet.listunspent(minconf=0, addresses=[addr])[0]
-        assert utxo["txid"] == txid and not utxo["solvable"]  # No satisfaction logic (yet)
+        assert utxo["txid"] == txid and utxo["solvable"]
 
     def run_test(self):
         self.log.info("Making a descriptor wallet")