entproto: Added PhpNamespace() message option

[eko]

I have to generate protos that will be used by a PHP project.

This pull request adds a new entproto.PhpNamespace() option that can be added to messages this way:

func (MessageWithPhpNamespace) Annotations() []schema.Annotation {
	return []schema.Annotation{
		entproto.Message(
			entproto.PackageName("io.entgo.apps.todo"),
			entproto.PhpNamespace("My\\Company\\Todo"),
		),
	}
}