Helm downloader registration handler

[fabianburth]

What this PR does / why we need it

Registering the Registration Handler of the Helm Downloader Handler allows to register the Helm Download Handler for other artifact types (aka. resource types) than helmChart, for which it is configured by default.

This configuration can be done through adding this section to the ocm config file (or passing such a config directly).

        type: downloader.ocm.config.ocm.software
        description: "my standard download handler configuration"
        handlers:
          - name: helm/artifact
            artifactType: NotHelmChart
            mimeType: application/vnd.oci.image.manifest.v1+json

Which issue(s) this PR fixes

[hilmarf]

I hope we don't lose control for so many different const values...
https://github.com/open-component-model/ocm/blob/main/cmds/ocm/commands/ocmcmds/common/inputs/types/helm/type.go#L7
https://github.com/open-component-model/ocm/blob/main/api/ocm/elements/artifactaccess/helmaccess/resource.go#L12
https://github.com/open-component-model/ocm/blob/main/api/ocm/extensions/artifacttypes/const.go#L16