go: Add support for struct enums

[svix-mman]

This is what the API will look like

// struct enum with fields
sourceIn := models.IngestSourceIn{
	Name: "Mendy",
	Type: models.IngestSourceInTypeCron,
	Config: models.CronConfig{
		Payload:  "Hello from space",
		Schedule: "0 0 0 * *",
	},
}

// struct enum without fields
uid := "very unique"
sourceIn := models.IngestSourceIn{
	Name: "Mendy",
	Uid: &uid
	Type: models.IngestSourceInTypeGenericWebhook,
}

// read a field
if cronConfig, ok := sourceIn.Config.(models.CronConfig); ok {
	println(cronConfig.Payload)
}

edit(jplatte): Closes https://github.com/svix/monorepo-private/issues/9981, closes https://github.com/svix/monorepo-private/issues/10059