Add an option in the Helm chart to enable Istio sidecar injection for the discovery Pod

[djannot]

Is your feature request related to a problem? Please describe.
When using Gloo Edge with Istio and mTLS is enforced, the discovery Pod needs to be able to communicate with the Upstreams using mTLS.
Manually injecting the Istio sidecar in the discovery Pod works, but it needs to be automated.

Describe the solution you'd like
Add an option in the Helm chart to enable Istio sidecar injection for the discovery Pod