s3: GCS provider: fix server side copy of files bigger than 5G

GCS gives NotImplemented errors for multi-part server side copies. The
threshold for these is currently set just below 5G so any files bigger
than 5G that rclone attempts to server side copy will fail.

This patch works around the problem by adding a quirk for GCS raising
--s3-copy-cutoff to the maximum. This means that rclone will never use
multi-part copies for files in GCS. This includes files bigger than
5GB which (according to AWS documentation) must be copied with
multi-part copy. However this seems to work with GCS.

See: https://forum.rclone.org/t/chunker-uploads-to-gcs-s3-fail-if-the-chunk-size-is-greater-than-the-max-part-size/44349/
See: https://issuetracker.google.com/issues/323465186

backend/s3/s3.go

@@ -3222,6 +3222,10 @@ func setQuirks(opt *Options) {
 		// https://github.com/rclone/rclone/issues/6670
 		useAcceptEncodingGzip = false
 		useAlreadyExists = true // returns BucketNameUnavailable instead of BucketAlreadyExists but good enough!
+		// GCS S3 doesn't support multi-part server side copy:
+		// See: https://issuetracker.google.com/issues/323465186
+		// So make cutoff very large which it does seem to support
+		opt.CopyCutoff = math.MaxInt64
 	default:
 		fs.Logf("s3", "s3 provider %q not known - please set correctly", opt.Provider)
 		fallthrough