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/init.cpp

@@ -1421,7 +1421,10 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     for (bool fLoaded = false; !fLoaded && !ShutdownRequested();) {
         node.mempool = std::make_unique<CTxMemPool>(node.fee_estimator.get(), mempool_check_ratio);
 
-        node.chainman = std::make_unique<ChainstateManager>(chainparams);
+        const ChainstateManager::Options chainman_opts{
+            chainparams,
+        };
+        node.chainman = std::make_unique<ChainstateManager>(chainman_opts);
         ChainstateManager& chainman = *node.chainman;
 
         const bool fReset = fReindex;