Add imagePullSecretName support to helm chart

[Dean-Coakley]

Proposed changes

Continuation of #1232 for stable release.

• Add imagePullSecretName so we can support Rancher App Catalog integration.
• imagePullSecrets remains for backwards compatibility, which will be removed in a later release.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto master
• I will ensure my PR is targeting the master branch and pulling from my branch from my own fork

[Dean-Coakley]

Some outstanding questions:

• I don't see any hard requirement for this to be included in 1.9.1 for the Rancher integration. @pleshakov Do you have any preference for this to be included or excluded in that release?
• Do we want to mark imagePullSecrets as deprecated? Or it will just be in 1.10 release notes?
  • If we do, should something like the following be added to all doc locations and values?: "DEPRECATED: To be removed in future release. Please migrate to "imagePullSecretName"

[pleshakov]

@Dean-Coakley

I don't see any no hard requirement for this to be included in 1.9.1 for the Rancher integration. @pleshakov Do you have any preference for this to be included or excluded in that release?

Are we launching Rancher integration any time soon? If we don't include this now, does it mean we will need to create another minor release like 1.9.2 for the rancher integration?

Do we want to mark imagePullSecrets as deprecated? Or it will just be in 1.10 release notes?

Yeah, I think it makes sense to add 👍

[Dean-Coakley]

@pleshakov While not strictly required as the changes will be prepared on a branch on https://github.com/nginxinc/helm3-charts it makes sense to have all pull requests there map closely to KIC release tag.

So it would be best to include this in 1.9.1 for consistency.

[mikestephen]

LGTM!