Introduce a named template to delete/unset helm key.

Summary

Helm supports the deletion of default keys using the null value:

https://helm.sh/docs/chart_template_guide/values_files/#deleting-a-default-key

This is however only applicable for key that are defined in default values.yaml of the chart (or any valuesFiles passed to a flux HelmRelease as they are merged by the controller).

We've started to use the following pattern leveraging "set-only-if" named template in some places as here for example, which is not very convenient nor explicit.

      foo: '{{ tuple "" false | include "set-only-if" }}'

We should introduce a template "delete-key" or "unset" that would ease such deletion:

      foo: '{{ include "unset" "" }}'