Include data_types key in cached schema

[rly]

Description

See NeurodataWithoutBorders/matnwb#351

If an extension includes a list of data types from another namespace, the cached extension does not specify that list of data types (and so includes all data types).

Checklist

• Have you ensured the bug was not already reported ?
• Have you included a brief and descriptive title?
• Have you included a clear description of the problem you are trying to solve?
• Have you included a minimal code snippet that reproduces the issue you are encountering?
• Have you checked our Contributing document?

[mavaylon1]

@rly I'll let you decide on the milestone.