protect annotation namespaces from conflicts

[ms-lolo]

we have an API for creating custom annotations now, in apps.annotations. the namespaces are private details that help annotation authors to collect groups of a certain kind, so it's important for us to make sure we don't have two groups creating a commands annotation namespace with different meanings. we should come up with a way to auto-scope namespaces, similar to our approach with service ids. the interfaces don't need to be super concise, because we don't expect a huge amount of annotation namespaces to be created, and their usage is typically in private details of a package.