BIP draft: Raw() as subscript for descriptors

Allowing arbitrary hex data to be wrapped in sh(), wsh(), or even within the TREE argument of a tr(KEY, TREE) descriptor enables the representation of currently inexpressible information.

Specifically, the absence of this feature limits the representation of non-standard redeem and witness scripts. This occurs because they can currently only be represented as top-level raw(HEX) descriptors, which retain only the output script information and lack the ability to preserve the actual script.

This work stemmed from discussions with @sipa and @achow101 about the legacy wallet migration path, particularly while investigating migrated descriptors that lacked information about the script they were supposedly migrated from.

Additionally, issue #24114, specifically https://github.com/bitcoin/bitcoin/issues/24114#issuecomment-1127978154, provides historical context and highlights other valid use cases enabled by this behavior change.