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.

首先閱讀 stackable-cockpit 中的 helm.rs 和 oci.rs，以了解目前的 repository 和 OCI 實作如何提供給呼叫端。將 OCI 特定程式碼移到 helm 模組之後，讓呼叫端不再依賴其實作細節，同時保留目前支援的安裝方法。