UniValue: add reserve member function

- Only reserve keys when the typ is of `VOBJ`.

src/univalue/include/univalue.h

@@ -70,6 +70,8 @@ public:
 
     size_t size() const { return values.size(); }
 
+    void reserve(size_t new_cap);
+
     void getObjMap(std::map<std::string,UniValue>& kv) const;
     bool checkObject(const std::map<std::string,UniValue::VType>& memberTypes) const;
     const UniValue& operator[](const std::string& key) const;

src/univalue/lib/univalue.cpp

@@ -240,3 +240,10 @@ const UniValue& UniValue::find_value(std::string_view key) const
     return NullUniValue;
 }
 
+void UniValue::reserve(size_t new_cap)
+{
+    values.reserve(new_cap);
+    if (typ == VOBJ) {
+        keys.reserve(new_cap);
+    }
+}