[enhancement]: Add tiny-cloud-style APK repository configuration

# Enhancement

Add a preferred `apk.repositories` cloud-config interface for configuring Alpine Linux APK repositories.

The existing `apk_repos` syntax is boolean-driven and limited to a single Alpine mirror plus an optional local repository. A list-based interface better reflects `/etc/apk/repositories`, supports multiple repository groups, and permits raw custom repository URLs.

Example:
```yaml
#cloud-config
apk:
repositories:
- base_url: https://cdn.alpinelinux.org
version: v3.24
repos: [main, community]
- https://packages.example.net/alpine/custom
```

apk_repos should remain supported for backwards compatibility. The new apk interface should be documented as the preferred form; any deprecation of apk_repos can be considered separately after adoption.

Note that this is already implemented in [tiny-cloud](https://gitlab.alpinelinux.org/alpine/cloud/tiny-cloud/-/blob/190b5cf89b555cdbb00e030ebea9c281d614bc97/docs/alpine-config.5.scd).

Start by comparing tiny-cloud's docs/alpine-config.5.scd with cloud-init's existing apk_repos handling. Trace how the apk cloud-config interface is parsed and documented, then verify that list-based repositories support grouped and raw URLs while apk_repos remains backward compatible.

Written by the indexing model from the issue text.