add node interface method for getting maximum mempool size

src/interfaces/node.h

@@ -162,6 +162,9 @@ public:
     //! Get mempool dynamic usage.
     virtual size_t getMempoolDynamicUsage() = 0;
 
+    //! Get mempool maximum memory usage.
+    virtual size_t getMempoolMaxUsage() = 0;
+
     //! Get header tip height and time.
     virtual bool getHeaderTip(int& height, int64_t& block_time) = 0;