[BIP Proposal] rawtr() Output Script Descriptors

[Jean Pablo]

The rawtr() descriptor has been in Bitcoin Core since 24.0, but it was
never specified in a BIP. Pieter noted this on the bips repo back in
March [0] and nobody picked it up, so I wrote a draft:

https://github.com/jeanpablojp/bips/blob/rawtr-descriptor/bip-rawtr.mediawiki

BIP 390 only allows musig() inside tr(), rawtr() or sp(), and two of
its test vectors are rawtr() descriptors, so there's a published BIP
depending on an expression no BIP defines. Outside the repo people
fill the blank by guessing: bdk-ffi's test suite cites rawtr as
BIP-386, which doesn't mention it.

The draft follows the shape of BIPs 384 and 385. Test vectors were
generated against Bitcoin Core master with getdescriptorinfo and
deriveaddresses. The first vector reuses the key from the first tr()
vector of BIP 386: same key, different script, because rawtr()
applies no tweak. The musig() vectors come from BIP 390 verbatim.

Scope is rawtr(KEY) only. The partial-descriptor ideas (rawnode(),
rawleaf()) have their own design discussion in bips PR 1721 and I'm
not touching them here.

On structure: Appendix B of BIP 380 has been one BIP per expression
group so far, and musig() and sp() both got their own numbers this
year, so I drafted it as a separate BIP rather than a new section in
386.

Feedback welcome.

[0] https://github.com/bitcoin/bips/pull/2050#issuecomment-3986205616