Allow configMapRef in PackageInstall values

**Describe the problem/challenge you have**
The `values` field in the PackageInstall allows only secrets for values files. The complete secret will be hidden in diffs. But not all config values are sensitive values. It would be nice to split the values into a combination of ConfigMap and Secrets to see the values that are not sensitive.

**Describe the solution you'd like**
Allow to fetch values from config maps.
Add a field `configMapRef` to `PackageInstallValues` and fetch use defined configmaps.
https://github.com/vmware-tanzu/carvel-kapp-controller/blob/866424d03cea80e5cb35598f2ab449defe007d87/cli/vendor/github.com/vmware-tanzu/carvel-kapp-controller/pkg/apis/packaging/v1alpha1/package_install.go#L78-L81

**Anything else you would like to add:**
I'm aware that this can be security risk if sensitive values are defined in config maps. This should be mentioned in the docs.

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