e.g. sh(addr(ADDR)) or sh(raw(HEX)) are invalid descriptors.
Making sh and wsh top level functions to return addr/raw descriptors when
the subscript inference fails.
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).
