refactor: share and use GenerateRandomKey helper

Making the `GenerateRandomKey` helper available to other modules via
key.{h.cpp} allows us to create random private keys directly at
instantiation of CKey, in contrast to the two-step process of creating
the instance and then having to call `MakeNewKey(...)`.

src/qt/test/addressbooktests.cpp

@@ -85,8 +85,7 @@ void TestAddAddressesToSendBook(interfaces::Node& node)
     }
 
     auto build_address = [&wallet]() {
-        CKey key;
-        key.MakeNewKey(true);
+        CKey key = GenerateRandomKey();
         CTxDestination dest(GetDestinationForKey(
             key.GetPubKey(), wallet->m_default_address_type));