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.

[kkkdeon]

@stefanprodan
Could you please review this PR?
We also need the same case.

[sc250024]

This is a related PR: #1789.

Extra containers on the main Deployment object is also nice to have. The PR above enables deployment of extra objects in general.