refactor: Create interfaces earlier during initialization

Add AppInitInterfaces function so wallet chain and chain client interfaces are
created earlier during initialization. This is needed in the next commit to
allow the gui splash screen to be able to register for wallet events through a
dedicated WalletClient interface instead managing wallets indirectly through
the Node interface. This only works if the wallet client interface is created
before the splash screen needs to use it.

src/init.h

@@ -52,6 +52,10 @@ bool AppInitSanityChecks();
  * @pre Parameters should be parsed and config file should be read, AppInitSanityChecks should have been called.
  */
 bool AppInitLockDataDirectory();
+/**
+ * Initialize node and wallet interface pointers. Has no prerequisites or side effects besides allocating memory.
+ */
+bool AppInitInterfaces(NodeContext& node);
 /**
  * Bitcoin core main initialization.
  * @note This should only be done after daemonization. Call Shutdown() if this function fails.