Add support to APIcast for end user rate limits

Add user id to auth calls in the APIcast gateway in order to make use of end user plans
https://github.com/3scale/apicast/issues/255

IMPORTANT: End user rate limits will be re-implemented in future, this should feature should only be used as an upstream and on understanding that it will need to be migrated to the new implementation in future.

UPDATE 02/Sept/2018:
The implementation of this policy (“Edge Limiting Policy”) is very flexible and powerful and can cover several use cases

Example Use Cases

• End user rate limiting: Rate limit by a value of parsed JWT using the subject ( {{ jwt.sub }}).
• TPS rate limiting: Limit allowed Transactions per Second (TPS).
• Global rate limits per service: Apply limits per service rather than per app.
• Concurrent connection limit: Set the number of concurrent connections allowed.