Fix #33 - created/changed as date-time

Created by: creckord

This fixes #33 (closed): it changes the created/changed properties of Listing to string/date-time (timestamp in ISO-8601 format, e.g. 2021-07-01T18:54:30Z). This is what the server already returns despite the spec currently defining int64.

Also a few additional fixes for the spec:

• Market.categories array was wrong (one array too many)
• Named enums, so generated clients have a better time with them
• Validity issues with $ref