Expose the provider v0.5.1 mirror_strategy field through the config module
so rpm and alpine(apk) remotes can select how upstream mirrors are chosen
(round_robin | least_conn) across base_url + mirrorlist. Completes the
mirror IaC surface started in #24.
- remote_rpm / remote_alpine object vars gain mirror_strategy = optional(string, null)
- pass mirror_strategy through to the corresponding remote resources
- provider constraint stays ~> 0.5 (already resolves v0.5.1, which adds the attr)
Additive and optional; no real remote config changes.