feat: Add a way to customize the 'managed-by' label

[jkremser]

By default this is a no-operation (so backward compatibility is guaranteed). If the .customManagedBy value is not empty, it will be used as a value for app.kubernetes.io/managed-by label. This is useful, when using this helm chart as a source for some other tools just for rendering the manifests, but not using the helm {install,upgrade,uninstall} or any other day-2 stuff.

helm template . --set customManagedBy=foo | grep managed-by | head -1
    app.kubernetes.io/managed-by: foo

helm template . | grep managed-by | head -1                                                                                                                                                           
    app.kubernetes.io/managed-by: Helm

Checklist

• [x] I have verified that my change is according to the deprecations & breaking changes policy
• [x] Commits are signed with Developer Certificate of Origin (DCO - learn more)
• [x] README is updated with new configuration values (if applicable) learn more
• ~[ ] A PR is opened to update KEDA core (repo) (if applicable, ie. when deployment manifests are modified)~ N/A