rate limiting: option to group endpoints

[dobegor]

Describe the enhancement you'd like to see

1. have an rps and burst parameters, warn user that in case of a split in nginx-ingress these will be applied to each path separately (addressed by nginx-ingress: warn that path level rate limit is applied separately for each path #147)
2. introduce a group parameter (string) which user can use to customise how endpoints are grouped in rate limiting - this is supported both in Ambassador and Traefik
3. if no group is specified at the global level, treat it as a common rate limit (all endpoints are in the same group)
4. if no group is specified at the path/operation level, treat it as a separate rate limit for this path/operation

Additional context
Add any other context, CLI input/output, etc. about the enhancement here.