Bff yarp proxy improvements

[Erwinvandervalk]

The wireup of YARP in the BFF is now simplified by relying more on the functionality
of the BFF. This has several advantages, aside from the simplification. For example,
tracing and service discovery are automatically wired up correctly.

Most of the for configuring the AccessTokenRequestTransform has been moved from the Map extension method and the DefaultMessageInvokerFactory into the extension method MapRemoteBffApiEndpoint.

Breaking changes:

Removed DefaultMessageInvokerFactory

The class DefaultMessageInvokerFactory and it's interface IHttpMessageInvokerFactory have been removed. This class was used to configure the message invoker and was a point for mocking. If you wish to inject a http handler for unit testing, you can inject a custom IForwarderHttpClientFactory. For example:

   // A Forwarder factory that forwards the messages to a message handler (which can be easily retrieved from a testhost)
    public class BackChannelHttpMessageInvokerFactory(HttpMessageHandler backChannel) 
        : IForwarderHttpClientFactory
    {
        public HttpMessageInvoker CreateClient(ForwarderHttpClientContext context) => 
            new HttpMessageInvoker(backChannel);
    }

   // Wire up the forwarder in your application's test host:
    services.AddSingleton<IForwarderHttpClientFactory>(
         new BackChannelHttpMessageInvokerFactory(_apiHost.Server.CreateHandler()));

Removed class DefaultHttpTransformerFactory and IHttpTransformerFactory.

This class was used to configure the default transforms. This has now been changed in favor of allowing you to inject a delegate that configures Yarp into the method MapRemoteBffApiEndpoint(). The default logic has been moved to DefaultTransformers.DirectProxyWithAccessToken(). If you wish to extend this logic, you can wrap this function in a different method and inject that into it.

            endpoints.MapRemoteBffApiEndpoint(
                "/local-path",
                _apiHost.Url(),
                context =>
                {
                    // do something custom: IE: copy request headers
                    context.CopyRequestHeaders = true;

                    // wire up the default transformer logic
                    DefaultTransformers.DirectProxyWithAccessToken("/local-path", context);
                })
                // Continue with normal BFF configuration, for example, allowing optional user access tokens
                .WithOptionalUserAccessToken();

If you used a custom implementation of IHttpTransformerFactory to change the default behavior of MapRemoteBffApiEndpoint(),
for example to add additional transforms, then you can now inject a custom delegate into the di container:

services.AddSingleton<BffYarpTransformBuilder>(CustomDefaultYarpTransforms);
//...
// This is an example of how to add a response header to ALL invocations of MapRemoteBffApiEndpoint()
private void CustomDefaultBffTransformBuilder(string localpath, TransformBuilderContext context)
{
    context.AddResponseHeader("added-by-custom-default-transform", "some-value");
    DefaultBffYarpTransformerBuilders.DirectProxyWithAccessToken(localpath, context);
}

Removed method RemoteApiEndpoint.Map(localpath, apiAddress).

The Map method was no longer needed as most of the logic had been moved to either the MapRemoteBffApiEndpoint and the DefaultTransformers. The map method also wasn't very explicit about what it did and a number of test scenario's tried to verify if it wasn't called wrongly. You are now expected to call the method MapRemoteBffApiEndpoint. This method now has a nullable parameter that allows you to inject your own transformers.

AccessTokenRetrievalContext properties are now typed

The LocalPath and ApiAddress properties are now typed. They used to be strings. If you rely on these, for example for implementing
a custom IAccessTokenRetriever, then you should adjust their usage accordingly.

    /// <summary>
    /// The locally requested path.
    /// </summary>
    public required PathString LocalPath { get; set; }

    /// <summary>
    /// The remote address of the API.
    /// </summary>
    public required Uri ApiAddress { get; set; }

Tasks:

• Add exploration test for YARP
• Simplify wire-up of yarp
• Added additional unit tests
• remove double //
• restore the AccessTokenRetrievalContext properties

fixes #1692
fixes #1693

[leastprivilege]

Thanks Erwin! This looks very good!

Could you also add a migration guide from the old extensibility points to the YARP ones?

[leastprivilege]

Didn't do a full code review (someone else must do that) - but changes look good to me.