Add missing `envoyFilters` templating

Motivation

The old "operatorful" Istio allowed our customers to create their own custom EnvoyFilter resource inline in the values.yaml via the istio.values.envoyFilters key. This did not get translated into our new charts.

Acceptance Criteria

• A top-level key is added to this chart: additionalEnvoyFilters
• The top-level key controls iteration over EnvoyFilter resources, creating one per filter