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.

Comienza leyendo helm.rs y oci.rs en stackable-cockpit para comprender cómo se exponen actualmente el repositorio y las implementaciones de OCI a los llamadores. Mueve el código específico de OCI detrás del módulo helm, de modo que los llamadores ya no dependan de sus detalles de implementación, preservando al mismo tiempo los métodos de instalación compatibles actualmente.