Merge #16546: External signer support - Wallet Box edition

f75e0c1edd doc: add external-signer.md (Sjors Provoost)
d4b0107d68 rpc: send: support external signer (Sjors Provoost)
245b4457cf rpc: signerdisplayaddress (Sjors Provoost)
7ebc7c0215 wallet: ExternalSigner: add GetDescriptors method (Sjors Provoost)
fc5da520f5 wallet: add GetExternalSigner() (Sjors Provoost)
259f52cc33 test: external_signer wallet flag is immutable (Sjors Provoost)
2655197e1c rpc: add external_signer option to createwallet (Sjors Provoost)
2700f09c41 rpc: signer: add enumeratesigners to list external signers (Sjors Provoost)
07b7c940a7 rpc: add external signer RPC files (Sjors Provoost)
8ce7767071 wallet: add ExternalSignerScriptPubKeyMan (Sjors Provoost)
157ea7c614 wallet: add external_signer flag (Sjors Provoost)
f3e6ce78fb test: add external signer test (Sjors Provoost)
8cf543f96d wallet: add -signer argument for external signer command (Sjors Provoost)
f7eb7ecc67 test: framework: add skip_if_no_external_signer (Sjors Provoost)
87a97941f6 configure: add --enable-external-signer (Sjors Provoost)

Pull request description:

  Big picture overview in [this gist](https://gist.github.com/Sjors/29d06728c685e6182828c1ce9b74483d).

  This PR lets `bitcoind` call an arbitrary command `-signer=<cmd>`, e.g. a hardware wallet driver,  where it can fetch public keys, ask to display an address, and sign a transaction (using PSBT under the hood).

  It's design to work with https://github.com/bitcoin-core/HWI, which supports multiple hardware wallets. Any command with the same arguments and return values will work. It simplifies the manual procedure described [here](https://github.com/bitcoin-core/HWI/blob/master/docs/bitcoin-core-usage.md).

  Usage is documented in [doc/external-signer.md](
  https://github.com/Sjors/bitcoin/blob/2019/08/hww-box2/doc/external-signer.md), which also describes what protocol a different signer binary should conform to.

  Use `--enable-external-signer` to opt in, requires Boost::Process:

  ```
  Options used to compile and link:
    with wallet     = yes
    with gui / qt   = no
    external signer = yes
  ```

  It adds the following RPC methods:
  * `enumeratesigners`: asks <cmd> for a list of signers (e.g. devices) and their master key fingerprint
  * `signerdisplayaddress <address>`:  asks <cmd> to display an address

  It enhances the following RPC methods:
  * `createwallet`: takes an additional `external_signer` argument and fetches keys from device
  * `send`: automatically sends transaction to device and waits

  Usage TL&DR:
  * clone HWI repo somewhere and launch `bitcoind -signer=../HWI/hwi.py`
  * check if you can see your hardware device: `bitcoin-cli enumeratesigners`
  * create wallet and auto import keys `bitcoin-cli createwallet "hww" true true "" true true true`
  * display address on device: `bitcoin-cli signerdisplayaddress ...`
  * to spend, use `send` RPC and approve transaction on device

  Prerequisites:
  - [x] #21127 load wallet flags before everything else
  - [x] #21182 remove mostly pointless BOOST_PROCESS macro

  Potentially useful followups:
  - GUI support: bitcoin-core/gui#4
  - bumpfee support
  - (automatically) verify (a subset of) keys on the device after import, through message signing

ACKs for top commit:
  laanwj:
    re-ACK f75e0c1edd

Tree-SHA512: 7db8afd54762295c1424c3f01d8c587ec256a72f34bd5256e04b21832dabd5dc212be8ab975ae3b67de75259fd569a561491945750492f417111dc7b6641e77f

build_msvc/bitcoin_config.h

@@ -50,8 +50,8 @@
 /* define if the Boost::Filesystem library is available */
 #define HAVE_BOOST_FILESYSTEM /**/
 
-/* define if the Boost::Process library is available */
-#define HAVE_BOOST_PROCESS /**/
+/* define if external signer support is enabled (requires Boost::Process) */
+#define ENABLE_EXTERNAL_SIGNER /**/
 
 /* define if the Boost::System library is available */
 #define HAVE_BOOST_SYSTEM /**/