Make DownloadScheme.isAuthSupported an abstract method

It is too easy to forget to override isAuthSupported in download
schemes that support but do not require authentication.  Make it
abstract so anyone adding a new download scheme has to make an
explicit choice about whether it should show up on changes that are
not world-readable.

Change-Id: Ifbc70c0d3b6eea08ceee726317381cabd3892e03

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/config/DownloadScheme.java

@@ -30,9 +30,7 @@ public abstract class DownloadScheme {
   public abstract boolean isAuthRequired();
 
   /** @return whether this scheme supports authentication */
-  public boolean isAuthSupported() {
-    return isAuthRequired();
-  }
+  public abstract boolean isAuthSupported();
 
   /** @return whether the download scheme is enabled */
   public abstract boolean isEnabled();