Is there a postman collection for the ODK Central API?
There is not a comprehensive collection.
The API is documented using API Blueprint: https://raw.githubusercontent.com/getodk/central-backend/master/docs/api.md
I haven't tried this myself but if you search for "Postman" at https://apiblueprint.org/tools.html, you will find many tools that go from API Blueprint to Postman.
If you do make a Postman collection, please do share it!