[MM-40935] Add OnInstall() plugin hook (#19499)

model/plugin_on_install_event.go

@@ -0,0 +1,9 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package model
+
+// OnInstallEvent is sent to the plugin when it gets installed.
+type OnInstallEvent struct {
+	UserId string // The user who installed the plugin
+}