Merge bitcoin/bitcoin#25218: refactor: introduce generic 'Result' class and connect it to CreateTransaction and GetNewDestination

111ea3ab711414236f8678566a7884d48619b2d8 wallet: refactor GetNewDestination, use BResult (furszy)
22351725bc4c5eb63ee45f607374bbf2d76e2b8c send: refactor CreateTransaction flow to return a BResult<CTransactionRef> (furszy)
198fcca162f4d2f877feab485e629ff89818ff56 wallet: refactor, include 'FeeCalculation' inside 'CreatedTransactionResult' (furszy)
7a45c33d1f8a758850cf8e7bd6ad508939ba5c0d Introduce generic 'Result' class (furszy)

Pull request description:

  Based on a common function signature pattern that we have all around the sources:
  ```cpp
  bool doSomething(arg1, arg2, arg3, arg4, &result_obj, &error_string) {
      // do something...
      if (error) {
          error_string = "something bad happened";
          return false;
      }

      result = goodResult;
      return true;
  }
  ```

  Introduced a generic class `BResult` that encapsulate the function boolean result, the result object (in case of having it) and, in case of failure, the string error reason.

  Obtaining in this way cleaner function signatures and removing boilerplate code:

  ```cpp
  BResult<Obj> doSomething(arg1, arg2, arg3, arg4) {
      // do something...
      if (error) return "something bad happened";

      return goodResult;
  }
  ```

  Same cleanup applies equally to the function callers' side as well. There is no longer need to add the error string and the result object declarations before calling the function:

  Before:
  ```cpp
  Obj result_obj;
  std::string error_string;
  if (!doSomething(arg1, arg2, arg3, arg4, result_obj, error_string)) {
      LogPrintf("Error: %s", error_string);
  }
  return result_obj;
  ```

  Now:
  ```cpp
  BResult<Obj> op_res = doSomething(arg1, arg2, arg3, arg4);
  if (!op_res) {
      LogPrintf("Error: %s", op_res.GetError());
  }
  return op_res.GetObjResult();
  ```

  ### Initial Implementation:

  Have connected this new concept to two different flows for now:

  1) The `CreateTransaction` flow. --> 7ba2b87c
  2) The `GetNewDestination` flow. --> bcee0912

  Happy note: even when introduced a new class into the sources, the amount of lines removed is almost equal to added ones :).

  Extra note: this work is an extended version (and a decoupling) of the work that is inside #24845 (which does not contain the `GetNewDestination` changes nor the inclusion of the `FeeCalculation` field inside `CreatedTransactionResult`).

ACKs for top commit:
  achow101:
    ACK 111ea3ab711414236f8678566a7884d48619b2d8
  w0xlt:
    reACK https://github.com/bitcoin/bitcoin/pull/25218/commits/111ea3ab711414236f8678566a7884d48619b2d8
  theStack:
    re-ACK 111ea3ab711414236f8678566a7884d48619b2d8
  MarcoFalke:
    review ACK 111ea3ab711414236f8678566a7884d48619b2d8 🎏

Tree-SHA512: 6d84d901a4cb923727067f25ff64542a40edd1ea84fdeac092312ac684c34e3688a52ac5eb012717d2b73f4cb742b9d78e458eb0e9cb9d6d72a916395be91f69

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