sylva-units: make it possible to control HelmRepository.spec

We need to make it possible to control the HelmRepository.spec of HelmRepositories produced by sylva-units:

introduce .Values.helmrepository_spec_default (with behavior similar to git_repo_spec_default or oci_repo_spec_default..)
introduce units.helmrepository_spec as an optional field to make it possible to override the spec of a HelmRepository for a given unit (usable only for a unit producing a HelmRepository)