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

internal/provider/models.go

@@ -6,6 +6,7 @@ type remoteAPI struct {
 	RepoType          string   `json:"repo_type,omitempty"`
 	BaseURL           string   `json:"base_url"`
 	Mirrorlist        []string `json:"mirrorlist,omitempty"`
+	MirrorStrategy    string   `json:"mirror_strategy,omitempty"`
 	Description       string   `json:"description,omitempty"`
 	Username          string   `json:"username,omitempty"`
 	Password          string   `json:"password,omitempty"`