2e5f7de wallet, rpc: update listdescriptors response format (Ivan Metlushko)

Pull request description:

  Update `listdescriptors` response format according to [RPC interface guidelines](https://github.com/bitcoin/bitcoin/blob/master/doc/developer-notes.md#rpc-interface-guidelines).

  This is a follow up for bitcoin#20226

  **Before:**
  ```
  Result:
  [                               (json array) Response is an array of descriptor objects
    {                             (json object)
      "desc" : "str",             (string) Descriptor string representation
      "timestamp" : n,            (numeric) The creation time of the descriptor
      "active" : true|false,      (boolean) Activeness flag
      "internal" : true|false,    (boolean, optional) Whether this is internal or external descriptor; defined only for active descriptors
      "range" : [                 (json array, optional) Defined only for ranged descriptors
        n,                        (numeric) Range start inclusive
        n                         (numeric) Range end inclusive
      ],
      "next" : n                  (numeric, optional) The next index to generate addresses from; defined only for ranged descriptors
    },
    ...
  ]
  ```

  **After:**
  ```
  Result:
  {                                 (json object)
    "wallet_name" : "str",          (string) Name of wallet this operation was performed on
    "descriptors" : [               (json array) Array of descriptor objects
      {                             (json object)
        "desc" : "str",             (string) Descriptor string representation
        "timestamp" : n,            (numeric) The creation time of the descriptor
        "active" : true|false,      (boolean) Activeness flag
        "internal" : true|false,    (boolean, optional) Whether this is internal or external descriptor; defined only for active descriptors
        "range" : [                 (json array, optional) Defined only for ranged descriptors
          n,                        (numeric) Range start inclusive
          n                         (numeric) Range end inclusive
        ],
        "next" : n                  (numeric, optional) The next index to generate addresses from; defined only for ranged descriptors
      },
      ...
    ]
  }
  ```

ACKs for top commit:
  achow101:
    re-ACK 2e5f7de
  meshcollider:
    utACK 2e5f7de
  jonatack:
    re-ACK 2e5f7de

Tree-SHA512: 49bf73e46e2a61003ce594a4bfc506eb9592ccb799c2909c43a1a527490a4b4009f78dc09f3d47b4e945d3d7bb3cd2632cf48c5ace5feed5066158cc010dddc1