feat(argo-cd): represent cluster credentials as a map

[nbarrientos]

Closes #2592.

Checklist:

• [x] I have bumped the chart version according to versioning
• [x] I have updated the documentation according to documentation
• [x] I have updated the chart changelog with all the changes that come with this pull request according to changelog.
• [ ] Any new values are backwards compatible and/or have sensible default.
• [x] I have signed off all my commits as required by DCO.
• [ ] My build is green (troubleshooting builds).

~I'll re-generate the docs and bump the version if this change request goes forward. This contains non-backwards compatible changes. The rationale for this change is in the issue it closes.~

Thanks for considering it.

[nbarrientos]

Similar breaking change recently accepted: https://github.com/argoproj/argo-helm/commit/237493a4ab1478a0c33cb1253767f65ce4ec007c

https://github.com/argoproj/argo-helm/issues/2523