Manifest emitting external flag and http2 transport information.

[mitchdenny]

This PR adds the ability for developers to mutate service bindings for specific publishers. Here is an example of the usage:

var app = builder.AddProject<Projects.Frontend>("frontend")
                            .WithServiceBindingForPublisher("manifest", "https", context => context.Binding.AsExternal());

When hitting F5 to debug locally, this would do nothing because the service bindings are derived from the launch profile. However when publishing a manifest the callback would be invoked and we can mark the binding as external. There is also AsHttp2() mutator as well. At this point in time the publisher argument is "stringy" but it could be made to be a generic as well.

[ellismg]

LGTM - I think the design makes sense and I see how we can use it to unblock the end to end we want to deliver, so thanks for the fix (and having the checked in manifest, so I can easily see it work).

Less confident in my ability to review the actual implementation and know if there are any gaps, because I just have no real mental model of how this stuff works internally yet, but from the diff everything seems reasonable.

As a consumer of the manifest, this LGTM.