Add support for NSX-T Edge Gateway Static Routes

[Didainius]

Added support for NSX-T Edge Gateway Static Route configuration via types

• types NsxtEdgeGatewayStaticRoute, types.NsxtEdgeGatewayStaticRoute
• methodsNsxtEdgeGateway.CreateStaticRoute, NsxtEdgeGateway.GetAllStaticRoutes, NsxtEdgeGateway.GetStaticRouteByNetworkCidr, NsxtEdgeGateway.GetStaticRouteByName, NsxtEdgeGateway.GetStaticRouteById, NsxtEdgeGatewayStaticRoute.Update, NsxtEdgeGatewayStaticRoute.Delete