Added support for cold start in image perf plugin

Summary:
This diff adds support for cold start in image perf plugin. Now developer can see which images have been requested as part of cold start.

CC: Diego Sanchez Vivian Wehner Alexander Oprisnik Gaurav Nijhara

Reviewed By: jknoxville

Differential Revision: D14227065

fbshipit-source-id: 3347b460e491b218115d8572cb8b323de909ef1b

src/plugins/fresco/api.js

@@ -58,6 +58,7 @@ export type ImageEvent = {
   startTime: Timestamp,
   endTime: Timestamp,
   source: string,
+  coldStart: number,
   viewport?: ViewportData, // not set for prefetches
 };