Merge #16546: External signer support - Wallet Box edition

f75e0c1edde39a91cc353b0102638e232def9476 doc: add external-signer.md (Sjors Provoost)
d4b0107d68a91ed4d1a5c78c8ca76251329d3f3c rpc: send: support external signer (Sjors Provoost)
245b4457cf9265190a05529a0a97e1cb258cca8a rpc: signerdisplayaddress (Sjors Provoost)
7ebc7c0215979c53b92a436acc8b5b607b8d735a wallet: ExternalSigner: add GetDescriptors method (Sjors Provoost)
fc5da520f5c72287f59823b8a6d748dda49c574a wallet: add GetExternalSigner() (Sjors Provoost)
259f52cc33817a00b91ec9c7d078c07b88db7ab4 test: external_signer wallet flag is immutable (Sjors Provoost)
2655197e1c2dea9536c32afe1482ced4a1f481e9 rpc: add external_signer option to createwallet (Sjors Provoost)
2700f09c4130af6167ce71f46960e92ca800e205 rpc: signer: add enumeratesigners to list external signers (Sjors Provoost)
07b7c940a7da138d55a484ef83fee19ebf58a867 rpc: add external signer RPC files (Sjors Provoost)
8ce7767071779a0170364e6426bd393ed71bf281 wallet: add ExternalSignerScriptPubKeyMan (Sjors Provoost)
157ea7c614950d61bfe405310e2aaabcee31f7a3 wallet: add external_signer flag (Sjors Provoost)
f3e6ce78fba2b31173fe7b606aa9edb5b615bff3 test: add external signer test (Sjors Provoost)
8cf543f96dcd6fdfac1367b9e2b1d7d51be8bb76 wallet: add -signer argument for external signer command (Sjors Provoost)
f7eb7ecc6750ab267a979d9268ce5b5d151c26de test: framework: add skip_if_no_external_signer (Sjors Provoost)
87a97941f667483bbf2ab00929e03a2199cb8a62 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 f75e0c1edde39a91cc353b0102638e232def9476

Tree-SHA512: 7db8afd54762295c1424c3f01d8c587ec256a72f34bd5256e04b21832dabd5dc212be8ab975ae3b67de75259fd569a561491945750492f417111dc7b6641e77f

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