Merge bitcoin/bitcoin#21500: wallet, rpc: add an option to list private descriptors

bb822a7af8 wallet, rpc: add listdescriptors private option (S3RK)

Pull request description:

  Rationale: make it possible to backup your wallet with `listdescriptors` command

  * The default behaviour is still to show public version
  * For private version only the root xprv is returned

  Example use-case:
  ```
  > bitcoin-cli -regtest -named createwallet wallet_name=old descriptors=true
  > bitcoin-cli -regtest -rpcwallet=old listdescriptors true | jq '.descriptors' > descriptors.txt

  > bitcoin-cli -regtest -named createwallet wallet_name=new descriptors=true blank=true
  > bitcoin-cli -regtest -rpcwallet=new importdescriptors "$(cat descriptors.txt)"
  ```

  In case of watch-only wallet without private keys there will be following output:
  ```
  error code: -4
  error message:
  Can't get descriptor string.
  ```

ACKs for top commit:
  achow101:
    re-ACK bb822a7af8
  Rspigler:
    tACK bb822a7af8
  jonatack:
    ACK bb822a7af8 per `git diff 2854ddc bb822a7`
  prayank23:
    tACK bb822a7af8
  meshcollider:
    Code review ACK bb822a7af8

Tree-SHA512: f6dddc72a74e5667071ccd77f8dce578382e8e29e7ed6a0834ac2e114a6d3918b59c2f194f4079b3259e13d9ba3b4f405619940c3ecb7a1a0344615aed47c43d

src/wallet/scriptpubkeyman.h

@@ -621,7 +621,7 @@ public:
     const WalletDescriptor GetWalletDescriptor() const EXCLUSIVE_LOCKS_REQUIRED(cs_desc_man);
     const std::vector<CScript> GetScriptPubKeys() const;
 
-    bool GetDescriptorString(std::string& out) const;
+    bool GetDescriptorString(std::string& out, const bool priv) const;
 
     void UpgradeDescriptorCache();
 };