Prefer:resolution/missing options should be inside a Content-Type

[steve-chavez]

Problem

Currently there are disparate prefers headers to do a bulk insert and upsert. Ideally this would be done through a single content type so it later can be overridden with #2826.

A similar idea was previously discussed on #2066 (comment)

Solution

Use a vendor media type that contains both options:

POST /employees
Content-Type: application/vnd.pgrst.bulk+json;resolution=merge-duplicates;missing=default

---

Later on we can include the on_conflict and columns query parameters as parameters of the same content-type.