Add read-only Engine (#33563)

This change adds an engine implementation that opens a reader on an
existing index but doesn't permit any refreshes or modifications
to the index.

Relates to #32867
Relates to #32844

Author: s1monw

server/src/main/java/org/elasticsearch/index/engine/Engine.java

@@ -661,7 +661,7 @@ protected final void ensureOpen() {
     }
 
     /** get commits stats for the last commit */
-    public CommitStats commitStats() {
+    public final CommitStats commitStats() {
         return new CommitStats(getLastCommittedSegmentInfos());
     }
 
@@ -951,7 +951,9 @@ public final boolean refreshNeeded() {
      *
      * @return the commit Id for the resulting commit
      */
-    public abstract CommitId flush() throws EngineException;
+    public final CommitId flush() throws EngineException {
+        return flush(false, false);
+    }
 
 
     /**

server/src/main/java/org/elasticsearch/index/engine/InternalEngine.java

@@ -1576,11 +1576,6 @@ public boolean shouldPeriodicallyFlush() {
             || localCheckpointTracker.getCheckpoint() == localCheckpointTracker.getMaxSeqNo();
     }
 
-    @Override
-    public CommitId flush() throws EngineException {
-        return flush(false, false);
-    }
-
     @Override
     public CommitId flush(boolean force, boolean waitIfOngoing) throws EngineException {
         ensureOpen();