WIXFEAT:4505 - WixHttpExtension for URL reservations

I tried to find all the places to add the new extension and implement everything, but I might have missed something.

I would like to port this back to v3.x.
Looks like the `ServiceDependency` element's `Group` attribute made you decide at compile time whether it would be a service group or not.  I don't think you should have to decide at compile time whether you're sending in a SID.