Introduce extensions for v1.1 (#1457)
Introduce extensions for v1.1 (#1457)

* Amend the client and server content negotiation responsibilities to include the `ext` parameter

* Clarify server responsibilities

* Introduce extension namespacing

* Add namespace example

Co-authored-by: Gabe Sullice <18a98c35f49808b45edadc75fb1b25ebfd4037d6@sullice.com>

json-api/json-api,json-api/json-api,json-api/json-api,json-api/json-api,json-api/json-api