redpanda: allow overriding (parts) of pod specs

[chrisseto]

Previously it was not possible to individually set annotations, labels, or securityContexts on the Pods of the post install/upgrade Jobs. The inability to do so may prevent certain users from being able to run our chart within their environment. Namely, those using istio or those with strict Pod security requirements.

This commit extends the pattern of statefulset.podTemplate to apply to both the post install and post upgrade jobs. It also adds the ability to set pod and container security contexts via the pod template by implementing a good enough version of strategic merge patch that primarily relies on sprig's merge.

Additionally, this commit deprecates a handful of fields that are now better served via podTemplate.

[chrisseto]

There's a couple failing tests that I need to clean up but I want to get some opinions before diving too much deeper into this.

I've been trying to avoid fully implementing SMP/JSON MP but I'm wondering if it would be better to go all in rather than a half baked approach.

@JakeSCahill , if you have any suggestions on how to better document this I'm all ears. I think I've been looking at it too long to properly explain it.

I also needed to check how the updated README is now rendering :)