Add support for custom path bases

[poke]

When you do app.UsePathBase("/foo") the whole app basically runs at the subpath /foo. So the sitemap at /foo/sitemap.xml should generate paths that respect that path base.

This would usually be super simple, but the middleware is registered using applicationBuilder.Map(…). And the way the MapMiddleware works is by adjusting the path base itself.

So we have to change the way we are matching the path and will likely have to do that within the middleware itself instead. But doing that will actually help us with #5 so we can serve different endpoints later.

[ctolkien]

Is there a better way to register this other than MapMiddleware ?