Schema: Complete _get_types implementation

[jpmckinney]

There is some commented-out code suggesting the JSON Schema validation properties that I intended to add support for. In addition to those, I can't remember if required fields are indicated.

We can compare the functionality / presentation to the sphinxcontrib-jsonschema Sphinx directive.

For a reason I forget, I noted the PPP extension as a good test.

[jpmckinney]

In terms of unifying the tables in the Extension Explorer with sphinxcontrib-jsonschema, see also the feature request in open-contracting/infrastructure#240