bitcoin

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Ava Chow a8b582ec1d Merge bitcoin/bitcoin#32784 : wallet: derivehdkey RPC to get xpub at arbitrary path

c3945bfd2b doc: use derivehdkey in multisig tutorial (Sjors Provoost)
3662e33669 test: use derivehdkey in M-of-N multisig demo (Sjors Provoost)
d9570f0838 rpc: add derivehdkey (Sjors Provoost)
62da9f9614 wallet: add GetExtKey helper (Sjors Provoost)
aaf1548475 wallet: generalize GetActiveHDPubKeys helper (Sjors Provoost)
3821452c4a refactor: add hardened derivation helper (Sjors Provoost)
0ab61caafd rpc: ParsePathBIP32 helper (Sjors Provoost)
e36c4b76e1 util: reject out-of-range BIP32 keypath indices (Sjors Provoost)
ba78c31a00 fuzz: check ParseHDKeypath/WriteHDKeypath round-trip (Sjors Provoost)
8cce969085 Have ParseHDKeypath handle h derivation marker (Sjors Provoost)
fc53077762 test: move parse_hd_keypath test to bip32_tests (Sjors Provoost)
dab525eb77 key: add DeriveExtKey() helper (Sjors Provoost)

Pull request description:

  Adds a `derivehdkey` RPC that returns an xpub, or optionally the xprv, at an arbitrary BIP32 path (with at least one hardened step), derived from a wallet HD key.

  The main use case is coordinating a multisig setup, where each participant shares an xpub derived at a hardened path (e.g. `m/87h/0h/0h`) distinct from their default single-signature descriptors. See the (updated) `doc/multisig-tutorial.md` and (updated) functional test to see how that workflow improves.

  The first commits are some helpful helpers:

  - _key: add DeriveExtKey() helper_ - performs the actual derivation
  - _test: move parse_hd_keypath test to bip32_tests_ - from `psbt_wallet_tests`
  - _Have ParseHDKeypath handle h derivation marker_
  - _util: reject out-of-range BIP32 keypath indices_ -  `ParseHDKeypath` would previously map overflowing values without `h` to hardened.
  - _fuzz: check ParseHDKeypath/WriteHDKeypath round-trip_
  - _rpc: ParsePathBIP32 helper_
  - _refactor: add hardened derivation helper_ - `HasHardenedDerivation()`, to enforce the "at least one hardened step" rule
  - _wallet: generalize GetActiveHDPubKeys helper_ - extracts code from `gethdkeys` which `derivehdkey` needs
  - _wallet: add GetExtKey helper_ - reconstruct an xprv from a wallet xpub (analog of `GetKey()`); behavior-preserving prep, also simplifies `gethdkeys`.

  Meat and potatoes:
  - _rpc: add derivehdkey_ - the RPC itself, plus the `UnusedKey` filter on `GetHDPubKeys` that drives key selection.
  - _test: use derivehdkey in M-of-N multisig demo_ - rewrites the functional multisig test to use the RPC and `<0;1>` syntax.
  - _doc: use derivehdkey in multisig tutorial_ - same for the prose tutorial.

ACKs for top commit:
  pseudoramdom:
    code review ACK c3945bfd2b
  achow101:
    ACK c3945bfd2b
  w0xlt:
    That being the case, ACK c3945bfd2b

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