Adding FileWillBeUploaded plugin hook (#9169)

* Adding file upload hook.

* Adding hook test for FileWillBeUploaded

* Some debugging fixes.

* Fix typo.

* Fixing double close

* Fix capitalization on docs.

plugin/interface_generator/main.go

@@ -355,6 +355,7 @@ func removeExcluded(info *PluginInterfaceInfo) *PluginInterfaceInfo {
 			"Implemented",
 			"LoadPluginConfiguration",
 			"ServeHTTP",
+			"FileWillBeUploaded",
 		}
 		for _, exclusion := range excluded {
 			if exclusion == item {