chore: Move OCI code into a helm module to hide implementation details.

We currently have helm.rs and oci.rs in stackable-cockpit. In the future we want to consolidate all repo-relevant code into a single module so that calling functions do not need to know about the underlying detail. This will become more relevant when:

• external dependencies no longer exclusively use repositories (as is currently the case for e.g. demos)
• other chart types such as .tgz archives are considered

N.B.

From the helm online help, installations can use:

1. By chart reference: helm install mymaria example/mariadb
2. By path to a packaged chart: helm install mynginx ./nginx-1.2.3.tgz
3. By path to an unpacked chart directory: helm install mynginx ./nginx
4. By absolute URL: helm install mynginx https://example.com/charts/nginx-1.2.3.tgz
5. By chart reference and repo url: helm install --repo https://example.com/charts/ mynginx nginx
6. By OCI registries: helm install mynginx --version 1.2.3 oci://example.com/charts/nginx

We currently implement 1, 5, 6.

Lies zunächst helm.rs und oci.rs in stackable-cockpit, um zu verstehen, wie das aktuelle Repository und die OCI-Implementierungen für Aufrufer bereitgestellt werden. Verschiebe den OCI-spezifischen Code hinter das helm-Modul, sodass Aufrufer nicht mehr von dessen Implementierungsdetails abhängen, und bewahre dabei die derzeit unterstützten Installationsmethoden.