Set X-Cache and X-Cache-Lookup headers for cache (#8794)

X-Cache sets cache status of HIT if object is
served from the disk cache, or MISS otherwise.
X-Cache-Lookup is set to HIT if object was found
in the cache even if not served (for e.g. if cache
 entry was invalidated by ETag verification)

cmd/http/headers.go

@@ -40,6 +40,12 @@ const (
 	Action             = "Action"
 )
 
+// Non standard S3 HTTP response constants
+const (
+	XCache       = "X-Cache"
+	XCacheLookup = "X-Cache-Lookup"
+)
+
 // Standard S3 HTTP request constants
 const (
 	IfModifiedSince   = "If-Modified-Since"