Indicate whether each download scheme supports authentication

isAuthSupported() defaults to isAuthRequired(), which is the value
used here, so no behavior change intended.

Explicitly specifying whether authentication is supported makes it
clearer which download schemes should be shown for changes that are
not visible to anonymous users.  Later we can stop providing a default
implementation in DownloadScheme to prevent new download schemes from
getting a wrong setting by forgetting to override the default.

Change-Id: If6f11bf906c58c721b0a9c39b9dbee48384b6e32

src/main/java/com/googlesource/gerrit/plugins/download/scheme/AnonymousHttpScheme.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/download/scheme/AnonymousHttpScheme.java b/src/main/java/com/googlesource/gerrit/plugins/download/scheme/AnonymousHttpScheme.java
index 905b0b6..1ea2cf0 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/download/scheme/AnonymousHttpScheme.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/download/scheme/AnonymousHttpScheme.java
@@ -71,6 +71,11 @@
     return false;
   }
 
+  @Override
+  public boolean isAuthSupported() {
+    return false;
+  }
+
   private static String ensureSlash(String in) {
     if (in != null && !in.endsWith("/")) {
       return in + "/";

src/main/java/com/googlesource/gerrit/plugins/download/scheme/GitScheme.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/download/scheme/GitScheme.java b/src/main/java/com/googlesource/gerrit/plugins/download/scheme/GitScheme.java
index 77b12b0..e185fdc 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/download/scheme/GitScheme.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/download/scheme/GitScheme.java
@@ -56,6 +56,11 @@
     return false;
   }
 
+  @Override
+  public boolean isAuthSupported() {
+    return false;
+  }
+
   private static String ensureSlash(String in) {
     if (in != null && !in.endsWith("/")) {
       return in + "/";

src/main/java/com/googlesource/gerrit/plugins/download/scheme/HttpScheme.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/download/scheme/HttpScheme.java b/src/main/java/com/googlesource/gerrit/plugins/download/scheme/HttpScheme.java
index b97a1bd..af75046 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/download/scheme/HttpScheme.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/download/scheme/HttpScheme.java
@@ -86,6 +86,11 @@
     return true;
   }
 
+  @Override
+  public boolean isAuthSupported() {
+    return true;
+  }
+
   private static String ensureSlash(String in) {
     if (in != null && !in.endsWith("/")) {
       return in + "/";

src/main/java/com/googlesource/gerrit/plugins/download/scheme/SshScheme.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/download/scheme/SshScheme.java b/src/main/java/com/googlesource/gerrit/plugins/download/scheme/SshScheme.java
index f662060..8253b66 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/download/scheme/SshScheme.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/download/scheme/SshScheme.java
@@ -79,6 +79,11 @@
     return true;
   }
 
+  @Override
+  public boolean isAuthSupported() {
+    return true;
+  }
+
   private static String ensureSlash(String in) {
     if (in != null && !in.endsWith("/")) {
       return in + "/";