Add topologySpreadConstraints to Deployment

[sgametrio]

Description of changes: When using a single replica, it can happen that the Node on which the aws-node-termination-handler Pod is running gets terminated. If this happens then, the Pod can't listen for the event and detect it to handle it gracefully.

Running a HA aws-node-termination-handler helps in dealing with this by reducing probability that all the replicas gets terminated at the same time, but doesn't guarantee that the replicas are scheduled on different nodes.

By introducing .Values.topologySpreadConstraints field we allow optional forcing of scheduling pods on different nodes, therefore reducing to ~0 the possibility that no Pods are available to listen for events.

Example of using topologySpreadConstraints

topologySpreadConstraints:
    - maxSkew: 1
      topologyKey: kubernetes.io/hostname
      whenUnsatisfiable: DoNotSchedule

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[github-actions]

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[cjerad]

Hi @sgametrio! Thanks for the PR. Please add the new value to the list in config/helm/aws-node-termination-handler/README.md.

[sgametrio]

Hi @cjerad Thanks for the comment. I updated the file as you suggested. Please feel free to suggest any re-wording or further changes.

[cjerad]

Thanks!