support for singletons

[toumorokoshi]

we need to resolve whether singletons is an acceptable pattern for a resource, or is an implicit behavior that doesn't need to be explicitly stated in the resource definition (i.e. implemented server-side).

singletons are described in google.aip.dev today: https://google.aip.dev/156, with some patterns that help identify it.

[toumorokoshi]

It is my personal opinion that we should not support singletons as a first class pattern, primarily because I don't see a scenario where singletons are a first-class concept in a resource.

The areas where a singleton differ are:

1. no create, list, or delete methods.
2. update and get operation directly on the sub-collection rather than an actual resource.

2 is problematic for a host of reasons (results in a non-uniform resource name pattern), so I argue it should be removed. a resource can get create / list / delete-able without necessarily being a singleton, so we don't need a specific identifier for that.