resource_remote: add mirror_strategy attribute

The provider exposed mirrorlist but not mirror_strategy, so the API's
mirror load-balancing strategy (round_robin/least_conn) was unreachable
via Terraform. Add mirror_strategy the same way mirrorlist was added,
scoped to rpm/deb/alpine remotes.

- schema: optional string mirror_strategy on the remote resource
- ValidateConfig: reject on non-rpm/deb/alpine types and validate the
  round_robin/least_conn enum at plan time
- wire model + modelToAPI/apiToModel (empty->null to avoid perpetual diff)
- datasource (computed) + README + rpm example
- unit tests: modelToAPI/apiToModel round-trip and ValidateConfig matrix

examples/resources/artifactapi_remote_rpm/main.tf

@@ -25,6 +25,10 @@ resource "artifactapi_remote_rpm" "almalinux" {
     "https://mirror.realcompute.io/almalinux",
   ]
 
+  # Load-balancing strategy across base_url + mirrorlist:
+  # "round_robin" (default) or "least_conn".
+  mirror_strategy = "least_conn"
+
   immutable_ttl = 0
   mutable_ttl   = 7200
 }