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/wallet/wallet.cpp

@@ -3578,8 +3578,7 @@ void CWallet::SetupDescriptorScriptPubKeyMans()
 
     if (!IsWalletFlagSet(WALLET_FLAG_EXTERNAL_SIGNER)) {
         // Make a seed
-        CKey seed_key;
-        seed_key.MakeNewKey(true);
+        CKey seed_key = GenerateRandomKey();
         CPubKey seed = seed_key.GetPubKey();
         assert(seed_key.VerifyPubKey(seed));