Add ChainstateManagerOpts, using as ::Options

[META] Although it seems like we don't need it for just one option,
       we're going to introduce another member to this struct *in the
       next commit*. In future patchsets for libbitcoinkernel decoupling
       it from ArgsManager, even more members will be added here.

src/bitcoin-chainstate.cpp

@@ -70,7 +70,10 @@ int main(int argc, char* argv[])
 
 
     // SETUP: Chainstate
-    ChainstateManager chainman{chainparams};
+    const ChainstateManager::Options chainman_opts{
+        chainparams,
+    };
+    ChainstateManager chainman{chainman_opts};
 
     auto rv = node::LoadChainstate(false,
                                    std::ref(chainman),