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/datasource_remote.go

@@ -31,6 +31,7 @@ func (d *remoteDataSource) Schema(_ context.Context, _ datasource.SchemaRequest,
 			"package_type":       schema.StringAttribute{Computed: true},
 			"base_url":           schema.StringAttribute{Computed: true},
 			"mirrorlist":         schema.ListAttribute{Computed: true, ElementType: types.StringType},
+			"mirror_strategy":    schema.StringAttribute{Computed: true},
 			"description":        schema.StringAttribute{Computed: true},
 			"immutable_ttl":      schema.Int64Attribute{Computed: true},
 			"mutable_ttl":        schema.Int64Attribute{Computed: true},
@@ -59,6 +60,7 @@ type remoteDataSourceModel struct {
 	PackageType       types.String `tfsdk:"package_type"`
 	BaseURL           types.String `tfsdk:"base_url"`
 	Mirrorlist        types.List   `tfsdk:"mirrorlist"`
+	MirrorStrategy    types.String `tfsdk:"mirror_strategy"`
 	Description       types.String `tfsdk:"description"`
 	ImmutableTTL      types.Int64  `tfsdk:"immutable_ttl"`
 	MutableTTL        types.Int64  `tfsdk:"mutable_ttl"`
@@ -110,6 +112,7 @@ func (d *remoteDataSource) Read(ctx context.Context, req datasource.ReadRequest,
 		PackageType:       types.StringValue(remote.PackageType),
 		BaseURL:           types.StringValue(remote.BaseURL),
 		Mirrorlist:        stringsToList(ctx, remote.Mirrorlist),
+		MirrorStrategy:    stringOrNull(remote.MirrorStrategy),
 		Description:       types.StringValue(remote.Description),
 		ImmutableTTL:      types.Int64Value(remote.ImmutableTTL),
 		MutableTTL:        types.Int64Value(remote.MutableTTL),