Add RPC/WS ports to server_info (#4427)

Enhance the /crawl endpoint by publishing WebSocket/RPC ports in the
server_info response. The function processing requests to the /crawl
endpoint actually calls server_info internally, so this change enables a
server to advertise its WebSocket/RPC port(s) to peers via the /crawl
endpoint. `grpc` and `peer` ports are included as well.

The new `ports` array contains objects, each containing a `port` for the
listening port (number string), and an array `protocol` listing the
supported protocol(s).

This allows crawlers to build a richer topology without needing to
port-scan nodes. For non-admin users (including peers), the info about
*admin* ports is excluded.

Also increase test coverage for RPC ServerInfo.

Fix #2837.

src/ripple/app/main/Application.cpp

@@ -602,6 +602,13 @@ public:
         return *m_networkOPs;
     }
 
+    virtual ServerHandlerImp&
+    getServerHandler() override
+    {
+        assert(serverHandler_);
+        return *serverHandler_;
+    }
+
     boost::asio::io_service&
     getIOService() override
     {