Added copy-with-generation-preconditions

Added copy-with-generation-preconditions to the convenience functions
section. The purpose of this function is to set some preconditions
before the copy takes place to avoid race conditions where files are
added just before the copy.

I can imagine the "Convenience functions" section becoming unwieldy.
There is probably a more composable design here which I'm happy to work
towards if anyone has feedback.

src/clj_gcloud/storage.clj

@@ -209,6 +209,20 @@
 ;; Convenience functions
 ;;
 
+(defn copy-with-generation-preconditions
+  "Copy blobs within cloud storage using preconditions recommended in Google Cloud Storage guides
+  See: https://cloud.google.com/storage/docs/copying-renaming-moving-objects#storage-copy-object-java"
+  [^Storage storage ^BlobId source-blob-id ^BlobId target-blob-id]
+  (let [precondition (if-let [target (.get storage target-blob-id)]
+                       (Storage$BlobTargetOption/generationMatch (.getGeneration target))
+                       (Storage$BlobTargetOption/doesNotExist))
+        builder (-> (Storage$CopyRequest/newBuilder)
+                    (.setSource source-blob-id)
+                    (.setTarget target-blob-id (into-array Storage$BlobTargetOption [precondition]))
+                    .build)]
+    (-> (.copy storage builder)
+        (.getResult))))
+
 (defn copy-file-to-storage
   "Convenience function for copying a local file to a blob to storage.
   By default the type is JSON encoded in UTF-8"