Add initial support for EgressGateways

Add initial support for egressgateways within the istio package.

We will want to provide support for these in a similar way to how we do this with ingress gateway values (although we do not need to support the "deprecated" path).

The supported values should include everything that is set for ingress gateways (which should match what is in the gatewayspec). We want this functionality to be DISABLED by default, pending further details on how we want to implement these by default. We should validate that deployment works when enabled

A basic guide of usage and setup is provided in the upstream docs, which may be helpful in testing to make sure things are deploying properly. You may need to add an additional Gateway via values to ensure that things function properly.