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

plugin/hooks.go

@@ -40,6 +40,7 @@ const (
 	OnWebSocketDisconnectID         = 22
 	WebSocketMessageHasBeenPostedID = 23
 	RunDataRetentionID              = 24
+	OnInstallID                     = 25
 	TotalHooksID                    = iota
 )
 
@@ -249,4 +250,12 @@ type Hooks interface {
 	//
 	// Minimum server version: 6.4
 	RunDataRetention(nowTime, batchSize int64) (int64, error)
+
+	// OnInstall is invoked after the installation of a plugin as part of the onboarding.
+	// It's called on every installation, not only once.
+	//
+	// In the future, other plugin installation methods will trigger this hook, e.g. an installation via the Marketplace.
+	//
+	// Minimum server version: 6.5
+	OnInstall(c *Context, event model.OnInstallEvent) error
 }