add support for oauth-proxy sidecar

[TheRealNoob]

Add the ability to add sidecars with the specific use-case in mind of oauth-proxy for the UI.

closes #82

[TheRealNoob]

one thing I think worth noting is that I nested extraContainers at a different level than extraVolumes. This is obviously not ideal, but I did it this way because I believe extraVolumes is located at the wrong level. Let me explain.

If I could be so forward, what I see implemented now a mixed presumption that this helm chart will only ever have one Pod and ServiceMonitor (because a SM can only point to a single pod). Some of the config says that statement is true and some says it's false. On the root of the values file are several items:

• podAnnotations
• podLabels
• podSecurityContext
• extraVolumes
• etc
  All of these objects can only be tied to one podSpec. If you add a second pod down the road, now these objects being located at the root is confusing. I think you lock yourself into a corner by having them there - I think the correct place is under the opencost object. An example of a second pod that you might add down the road could be subcharting Prometheus, in which case that would be controlled under the root prometheus object. I'm tempted to say that everything written now should be moved under opencost. Even things like ServiceAccount, which isn't technically 1:1, but if you're going to create another pod, it probably won't need the same k8s permissions as opencost, so having a different serviceAccount for it makes sense IMO.

This is all just my internal thoughts as I was drafting it. If you want me to move things around let me know, I won't object.

[github-actions]

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[mattray]

I don't have a means to test this, but if someone could validate this works for them we'll get it merged.

[debMan]

Works fine. I just tested and confirmed the functionality. Except for the chart version, everything works fine. I appreciate it if you @mattray merge this, need these sidecars.
Thanks in advanced.