Add option to specify additional containers in helm chart

[thechristschn]

This adds an option to specify additional containers in the flagger deployment.

We need this option to make adjustments to the istio-proxy sidecar, but it is build in a generic way to support all kinds of additional containers.

For example:

additionalContainers:
- name: istio-proxy
  image: auto
  resources:
    limits:
      memory: 1Gi

would insert the istio-proxy container definition at the end of the container list in the deployment.