artifactapi module: support mirrorlist on rpm/apk(alpine) remotes

Expose the provider v0.5.0 mirrorlist field through the config module so
rpm and alpine(apk) remotes can declare extra upstream mirror base URLs
(load-balanced with failover across base_url + mirrorlist).

- remote_rpm / remote_alpine object vars gain mirrorlist = optional(list(string), [])
- pass mirrorlist through to the corresponding remote resources
- bump artifactapi provider constraint to ~> 0.5

Additive and optional; no real remote config changes (deferred follow-up).

modules/artifactapi/variables.tf

@@ -8,6 +8,7 @@ variable "remote_alpine" {
     immutable_patterns = optional(list(string), [])
     mutable_patterns   = optional(list(string), [])
     stale_on_error     = optional(bool, true)
+    mirrorlist         = optional(list(string), [])
   }))
   default = {}
 }
@@ -78,6 +79,7 @@ variable "remote_rpm" {
     immutable_patterns = optional(list(string), [])
     mutable_patterns   = optional(list(string), [])
     stale_on_error     = optional(bool, true)
+    mirrorlist         = optional(list(string), [])
   }))
   default = {}
 }