This contains a proposal for a catalog API, provided access to the internal
contents of a registry instance. The API endpoint is prefixed with an
underscore, which is illegal in images names, to prevent collisions with
repositories names. To avoid issues with large result sets, a paginated version
of the API is proposed. We make an addition to the tags API to support
pagination to ensure the specification is conistent.
Signed-off-by: Stephen J Day <stephen.day@docker.com>