feat: add cable diagnostics command

https://telecominfraproject.atlassian.net/browse/OLS-246

Summary of changes:
- Added `cablediagnostics` command.

Signed-off-by: Ivan Chvets <ivan.chvets@kinarasystems.com>

src/RESTObjects/RESTAPI_GWobjects.cpp

@@ -808,4 +808,15 @@ namespace OpenWifi::GWObjects {
 		}
 		return false;
 	}
+
+	bool CableDiagnostics::from_json(const Poco::JSON::Object::Ptr &Obj) {
+		try {
+			field_from_json(Obj, "serial", serialNumber);
+			field_from_json(Obj, "when", when);
+			field_from_json(Obj, "ports", ports);
+			return true;
+		} catch (const Poco::Exception &E) {
+		}
+		return false;
+	}
 } // namespace OpenWifi::GWObjects