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/main.tf

@@ -10,6 +10,7 @@ resource "artifactapi_remote_alpine" "this" {
   mutable_patterns   = each.value.mutable_patterns
   stale_on_error     = each.value.stale_on_error
   mirrorlist         = each.value.mirrorlist
+  mirror_strategy    = each.value.mirror_strategy
 }
 
 resource "artifactapi_remote_docker" "this" {
@@ -76,6 +77,7 @@ resource "artifactapi_remote_rpm" "this" {
   mutable_patterns   = each.value.mutable_patterns
   stale_on_error     = each.value.stale_on_error
   mirrorlist         = each.value.mirrorlist
+  mirror_strategy    = each.value.mirror_strategy
 }
 
 resource "artifactapi_remote_pypi" "this" {