artifactapi module: plumb mirror_strategy on rpm/apk(alpine) remotes

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.

modules/artifactapi/variables.tf

@@ -9,6 +9,7 @@ variable "remote_alpine" {
     mutable_patterns   = optional(list(string), [])
     stale_on_error     = optional(bool, true)
     mirrorlist         = optional(list(string), [])
+    mirror_strategy    = optional(string, null)
   }))
   default = {}
 }
@@ -80,6 +81,7 @@ variable "remote_rpm" {
     mutable_patterns   = optional(list(string), [])
     stale_on_error     = optional(bool, true)
     mirrorlist         = optional(list(string), [])
+    mirror_strategy    = optional(string, null)
   }))
   default = {}
 }