Deprecate (and mark for removal) HttpContextExtensionService

The HttpContextExtensionService is effectively unused in platform and
uses the abandoned HttpService it is API.

We therefore should deprecate and remove that so the
org.eclipse.equinox.http.registry bundle becomes free from any API and
only contains internal implementation what would make us free to change
how it works,

bundles/org.eclipse.equinox.http.registry/src/org/eclipse/equinox/http/registry/HttpContextExtensionService.java

@@ -27,6 +27,7 @@
  * @noimplement This interface is not intended to be implemented by clients.
  * @noextend This interface is not intended to be extended by clients.
  */
+@Deprecated(forRemoval = true)
 public interface HttpContextExtensionService {
 	/**
 	 * returns the HttpContext associated with the HttpService reference and http