Adjust version extension signature (elastic#96898)

Now that TransportVersion.current() is lazily evaluated, version
extensions can return fully formed TransportVersion objects. This commit
adjusts the signature of the extension to return TransportVersion
instead of its id. It also adds some validation that the version
returned doesn't try to downgrade to a version lower than what is known
in server constants.

Author: rjernst

server/src/main/java/org/elasticsearch/TransportVersion.java

@@ -152,7 +152,9 @@ private static TransportVersion findCurrent(TransportVersion fallback) {
             if (versionExtension == null) {
                 return fallback;
             }
-            return new TransportVersion(versionExtension.getCurrentTransportVersionId());
+            var version = versionExtension.getCurrentTransportVersion();
+            assert version.onOrAfter(fallback);
+            return version;
         }
     }
 

server/src/main/java/org/elasticsearch/internal/VersionExtension.java

@@ -8,18 +8,20 @@
 
 package org.elasticsearch.internal;
 
+import org.elasticsearch.TransportVersion;
+
 import java.util.ServiceLoader;
 
 /**
  * Allows plugging in current version elements.
  */
 public interface VersionExtension {
     /**
-     * Returns the version id of the current transport version.
-     * Note this cannot return a TransportVersion object because it needs to be
-     * called during static initialization of TransportVersion.
+     * Returns the {@link TransportVersion} that Elasticsearch should use.
+     * <p>
+     * This must be at least equal to the latest version found in {@link TransportVersion} V_* constants.
      */
-    int getCurrentTransportVersionId();
+    TransportVersion getCurrentTransportVersion();
 
     /**
      * Loads a single VersionExtension, or returns {@code null} if none are found.