Support .NET 5.0

• Support NET 5.0
• Fix issues #115 , #116

Breaking change:

• If you are using lazy loading scenario you need to provide BaseServerAddress during configuration

Example:

            builder.Services.AddFlexGrid(
                cfg =>
                {
                    cfg.ApplyConfiguration(new WeatherForecastGridConfiguration());
                    cfg.ApplyConfiguration(new CustomerGridConfiguration());
                    cfg.ApplyConfiguration(new OrderGridConfiguration());
                },
                options =>
                {
                    options.IsServerSideBlazorApp = false;
                    options.UseAuthorizationForHttpRequests = true;
                    options.BaseServerAddress = builder.HostEnvironment.BaseAddress;
                }
            )