Make sidecar container image configurable in Helm Chart

[tafkam]

Is your feature request related to a problem? Please describe.
Self-hosted container registries are a requirement in air-gapped environments. It seems that for the webhook-injected sidecar proxy container this is hardcoded in https://github.com/Azure/azure-workload-identity/blob/main/pkg/webhook/webhook.go#L27-L29 using the value from the makefile.

Describe the solution you'd like
Support changing the the sidecar proxy registry and image with values defined in the Helm chart. This could be achieved by using environment variables in the webhook itself.

Describe alternatives you've considered

Additional context