[helm] Add volume management, custom labels, environment variables and custom existing secret

[pauldn-wttj]

Hello, 👋

Thanks for your work on this operator, it's very useful!
We installed it on our EKS cluster using Helm after some tweaking to make it work with AWS-flavored Kubernetes components (most notably the CSI driver for AWS secrets and the CNI), here they are in this PR.

Context

• When using a secrets CSI driver such as AWS CSI driver, we want to be able to mount volumes with secrets coming from a SecretProviderClass (see documentation), which will create a secret with some needed keys.
• When using AWS SecurityGroupPolicy, we want to be able to add custom labels onto our pods since it applies to pods with a matchLabels (see documentation)

Changelog

• Add volumes and volumeMounts in Helm chart, to declare and mount volumes on the pod.
• Add podLabels to add custom labels on the pod.
• Add env to specify additional environment variables injected in pod.
• Add existingSecret to specify an already existing secret where the operator may fetch its environment variables (this will not create a new secret).
• Add tolerations and nodeSelector to make it possible to manage more precisely where the pods are scheduled.

I am unsure as to how to bump the version.

Let me know if there is anything incorrect, I'll be happy to update it.

Thanks!

[hitman99]

Nice work