Provide example egress gateway configuration

Motivation

An egress gateway allows SREs to have more fine-grained control and observability of north-south traffic leaving their cluster by sending all of it through a single gateway. By providing an example egress gateway configuration, the Big Bang developers help build confidence in SREs attempting to create their own with big bang.

Acceptance Criteria

• An example egress gateway configuration must be provided as an overlay values
• Documentation must be written describing how the overlay works and how to bind traffic to the generated gateways