Support for Topology Spread Constraints feature.

[mshanmu]

This PR adds support for Topology Spread Constraints feature. The Topology Spread constraints specification allows the pods to be spread across your cluster among multiple failure-domains.

@nitisht

[dvaldivia]

This looks great, I'll test it and add some comments.

Since this is a 1.19 stable feature that requires a special flag before 1.18 per the documentation (included below) we must ensure this works on other versions, namely 1.17 and 1.18

Note: In versions of Kubernetes before v1.18, you must enable the EvenPodsSpread feature gate on the API server and the scheduler in order to use Pod topology spread constraints.

[mshanmu]

Thanks, @dvaldivia !! Please let me know if I need to do anything else.

[dvaldivia]

Tested with k8s 1.17 and 1.21. On 1.17 the topology gets removed from the statefulset but I didn't observe any re-sync of the statefulset from our operator, which might indicate either no problem, or no changes detected (which should happen)

[mshanmu]

@dvaldivia , Can you link this PR with the #666 issue? I cannot seem to assign this issue to myself and close it.