Add pipeline config variables to v2 workspaces API resource

As per this deprecation notice the /2.0/teams/ endpoints in the API will be removed on 14 October 2020.

The notice says they have been replaced by /2.0/workspaces/ endpoints, but those don’t include /pipelines_config/variables so users will have no way to get or set workspace variables via REST.

To preserve REST access to workspace variables we need the missing endpoint added.

Steps to Reproduce

Change integrations to use /2.0/workspaces/{id}/pipelines_config/variables/
instead of /2.0/teams/{id}/pipelines_config/variables/

Expected Results

Requests and responses work as before.

Actual Results

Requests return 404 Not Found.