resource_remote: add mirrorlist attribute (rpm/deb/apk remotes)

Expose the artifactapi remotes API's new mirrorlist field so users can
configure extra upstream mirror base URLs. The API load-balances
base_url + mirrorlist with failover.

- Add optional mirrorlist list attribute to the remote resource schema
  and the remote data source (read-only).
- Wire it end-to-end: TF model, wire model (json mirrorlist,omitempty),
  modelToAPI/apiToModel, and preserve null/empty list semantics to avoid
  plan churn.
- Scope to rpm/deb/alpine via ValidateConfig (plan-time error on other
  remote types); API 400 is the backstop.
- Docs + rpm example; unit tests for round-trip, null handling, and the
  type-scoping validation.

examples/resources/artifactapi_remote_rpm/main.tf

@@ -18,6 +18,13 @@ resource "artifactapi_remote_rpm" "almalinux" {
   base_url    = "https://gsl-syd.mm.fcix.net/almalinux"
   description = "AlmaLinux RPM package repository"
 
+  # Extra mirrors; requests are load-balanced with failover across
+  # base_url + mirrorlist (rpm/deb/alpine remotes only).
+  mirrorlist = [
+    "https://mirror.aarnet.edu.au/pub/almalinux",
+    "https://mirror.realcompute.io/almalinux",
+  ]
+
   immutable_ttl = 0
   mutable_ttl   = 7200
 }