imagekit: add mtime to the available metadata

The backend declares ReadMetadata but did not return the standard
mtime key, so the modification time was missing from the metadata.

backend/imagekit/imagekit.go

@@ -32,6 +32,12 @@ const (
 )
 
 var systemMetadataInfo = map[string]fs.MetadataHelp{
+	"mtime": {
+		Help:     "Time of last modification, read from the file's updatedAt field",
+		Type:     "RFC 3339",
+		Example:  "2006-01-02T15:04:05.999999999Z07:00",
+		ReadOnly: true,
+	},
 	"btime": {
 		Help:     "Time of file birth (creation) read from Last-Modified header",
 		Type:     "RFC 3339",
@@ -759,6 +765,7 @@ func uploadFile(ctx context.Context, f *Fs, in io.Reader, srcRemote string, opti
 // Metadata returns the metadata for the object
 func (o *Object) Metadata(ctx context.Context) (metadata fs.Metadata, err error) {
 
+	metadata.Set("mtime", o.file.UpdatedAt.Format(time.RFC3339Nano))
 	metadata.Set("btime", o.file.CreatedAt.Format(time.RFC3339))
 	metadata.Set("size", strconv.FormatUint(o.file.Size, 10))
 	metadata.Set("file-type", o.file.FileType)