rpc: decode Miniscript descriptor when possible in decodescript

The descriptor inference logic would previously always use a dummy
signing provider and would never analyze the witness script of a P2WSH
scriptPubKey.

Note even a valid Miniscript might not always be decodable from Script
without more contextual information (for instance the key preimage for a
pk_h).

test/functional/data/rpc_decodescript.json

@@ -69,7 +69,7 @@
             "p2sh": "2N34iiGoUUkVSPiaaTFpJjB1FR9TXQu3PGM",
             "segwit": {
                 "asm": "0 96c2368fc30514a438a8bd909f93c49a1549d77198ccbdb792043b666cb24f42",
-                "desc": "addr(bcrt1qjmprdr7rq522gw9ghkgfly7yng25n4m3nrxtmdujqsakvm9jfapqk795l5)#5akkdska",
+                "desc": "wsh(raw(02eeee))#gtay4y0z",
                 "hex": "002096c2368fc30514a438a8bd909f93c49a1549d77198ccbdb792043b666cb24f42",
                 "address": "bcrt1qjmprdr7rq522gw9ghkgfly7yng25n4m3nrxtmdujqsakvm9jfapqk795l5",
                 "type": "witness_v0_scripthash",