Merge bitcoin/bitcoin#23005: multiprocess: Delay wallet client construction

ad085f9ba1 multiprocess: Delay wallet client construction (Russell Yanofsky)

Pull request description:

  Delay wallet client construction until after logging, thread and other init for two reasons:

  - More responsive multiprocess GUI startup. When bitcoin-gui is started this moves the call from bitcoin-gui to bitcoin-node that spawns bitcoin-wallet off of the GUI event thread and onto the background GUI init executor thread.

  - Avoids feature_logging.py test failures with bitcoin-node by making bitcoin-wallet logging start after bitcoin-node logging starts,
    because the tests are not written to handle the bitcoin-wallet logging init code running first.

  This partially reverts commit b266b3e0bf, moving wallet client creation back to the place it was located before.

  ---

  This PR is part of the [process separation project](https://github.com/bitcoin/bitcoin/projects/10).

ACKs for top commit:
  laanwj:
    code review ACK ad085f9ba1
  hebasto:
    ACK ad085f9ba1, I have reviewed the code and it looks OK.

Tree-SHA512: 74d957ce2ee096db745c517124f60800185814b06c20db676090e10dce1b90311adbab02865a69731f8c39b9365f9ee14be0830ca1368cac9b474801ea92bad5

src/interfaces/node.h

@@ -206,6 +206,10 @@ public:
     using ShowProgressFn = std::function<void(const std::string& title, int progress, bool resume_possible)>;
     virtual std::unique_ptr<Handler> handleShowProgress(ShowProgressFn fn) = 0;
 
+    //! Register handler for wallet client constructed messages.
+    using InitWalletFn = std::function<void()>;
+    virtual std::unique_ptr<Handler> handleInitWallet(InitWalletFn fn) = 0;
+
     //! Register handler for number of connections changed messages.
     using NotifyNumConnectionsChangedFn = std::function<void(int new_num_connections)>;
     virtual std::unique_ptr<Handler> handleNotifyNumConnectionsChanged(NotifyNumConnectionsChangedFn fn) = 0;