Extract type schema

[mcupak]

To better support https://github.com/ga4gh-discovery/ga4gh-service-registry/issues/94, it would be useful to extract the definition of type to a separate schema, so that we can refer to it directly instead of duplicating. I think such reusable schema will be useful for other APIs as well.

This is an internal change not affecting the API, and as such I'd like to add it to 1.0.

[mcupak]

@andrewyatz WDYT?

[andrewyatz]

Makes sense to me