Ensuring required services are registered when using the server reflection

[danielvieiravega]

This PR solves the problem related in this issue.

It is being checked whether the services needed to use gRPC reflection have been added. If not, an InvalidOperationException is thrown, asking to call the IServiceCollection.AddGrpcReflection() method.

[JamesNK]

This is great. Exactly how I was thinking of improving the error message.

I'd like to have a unit test of this failing. There is an example of doing this here:

grpc-dotnet/test/Grpc.AspNetCore.Server.Tests/GrpcEndpointRouteBuilderExtensionsTests.cs

Lines 37 to 49 in 713935f

	[Test]
	public void MapGrpcService_WithoutServices_RaiseError()
	{
	// Arrange
	var services = new ServiceCollection();
	var routeBuilder = CreateTestEndpointRouteBuilder(services.BuildServiceProvider(validateScopes: true));
	// Act & Assert
	var ex = Assert.Throws<InvalidOperationException>(() => routeBuilder.MapGrpcService<Greeter.GreeterBase>())!;
	Assert.AreEqual("Unable to find the required services. Please add all the required services by calling " +
	"'IServiceCollection.AddGrpc' inside the call to 'ConfigureServices(...)' in the application startup code.", ex.Message);
	}

[danielvieiravega]

I added the test @JamesNK.

[JamesNK]

Perfect. Thanks!