Enable skip methods on retrying inputstreams

Currently we have a number of input streams that specifically override
the skip() method disabling the ability to skip bytes. In each case the
skip implementation works as we have properly implemented the
read(byte[]) methods used to discard bytes. However, we appear to have
disabled it as it would be possible to retry from the end of a skip if
there is a failure in the middle. At this time, that optimization is not
really necessary, however, we sporadically used skip so it would be nice
for the IS to support the method. This commit enables the super.skip()
and adds a comment about future optimizations.

modules/repository-azure/src/main/java/org/elasticsearch/repositories/azure/AzureBlobStore.java

@@ -765,11 +765,6 @@ public void close() {
             }
         }
 
-        @Override
-        public long skip(long n) {
-            throw new UnsupportedOperationException("skip is not supported");
-        }
-
         private void releaseByteBuf(ByteBuf buf) {
             ReferenceCountUtil.safeRelease(buf);
             this.byteBuf = null;

modules/repository-gcs/src/main/java/org/elasticsearch/repositories/gcs/GoogleCloudStorageRetryingInputStream.java

@@ -246,8 +246,10 @@ public void close() throws IOException {
     }
 
     @Override
-    public long skip(long n) {
-        throw new UnsupportedOperationException("GoogleCloudStorageRetryingInputStream does not support seeking");
+    public long skip(long n) throws IOException {
+        // This could be optimized on a failure by re-opening stream directly to the preferred location. However, it is rarely called,
+        // so for now we will rely on the default implementation which just discards bytes by reading.
+        return super.skip(n);
     }
 
     @Override

modules/repository-s3/src/main/java/org/elasticsearch/repositories/s3/S3RetryingInputStream.java

@@ -225,8 +225,10 @@ private void maybeAbort(S3ObjectInputStream stream) {
     }
 
     @Override
-    public long skip(long n) {
-        throw new UnsupportedOperationException("S3RetryingInputStream does not support seeking");
+    public long skip(long n) throws IOException {
+        // This could be optimized on a failure by re-opening stream directly to the preferred location. However, it is rarely called,
+        // so for now we will rely on the default implementation which just discards bytes by reading.
+        return super.skip(n);
     }
 
     @Override

modules/repository-url/src/main/java/org/elasticsearch/common/blobstore/url/http/RetryingHttpInputStream.java

@@ -112,8 +112,10 @@ public int read(byte[] b, int off, int len) throws IOException {
     }
 
     @Override
-    public long skip(long n) {
-        throw new UnsupportedOperationException("RetryingHttpInputStream does not support seeking");
+    public long skip(long n) throws IOException {
+        // This could be optimized on a failure by re-opening stream directly to the preferred location. However, it is rarely called,
+        // so for now we will rely on the default implementation which just discards bytes by reading.
+        return super.skip(n);
     }
 
     @Override