feat(exp): add labelutils with Selector

This method is very helpful if you want to create resources and later
clean them up with the same label set. Creating resources requires a
`map[string]string`, but filtering resource lists takes a selector
string.

hcloud/exp/labelutils/selector.go

@@ -0,0 +1,24 @@
+package labelutils
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+)
+
+// Selector combines the label set into a [label selector](https://docs.hetzner.cloud/#label-selector) that only selects
+// resources have all specified labels set.
+//
+// The selector string can be used to filter resources when listing, for example with [hcloud.ServerClient.AllWithOpts()].
+func Selector(labels map[string]string) string {
+	selectors := make([]string, 0, len(labels))
+
+	for k, v := range labels {
+		selectors = append(selectors, fmt.Sprintf("%s=%s", k, v))
+	}
+
+	// Reproducible result for tests
+	sort.Strings(selectors)
+
+	return strings.Join(selectors, ",")
+}

hcloud/exp/labelutils/selector_test.go

@@ -0,0 +1,39 @@
+package labelutils
+
+import "testing"
+
+func TestSelector(t *testing.T) {
+	tests := []struct {
+		name             string
+		labels           map[string]string
+		expectedSelector string
+	}{
+		{
+			name:             "empty selector",
+			labels:           map[string]string{},
+			expectedSelector: "",
+		},
+		{
+			name:             "single label",
+			labels:           map[string]string{"foo": "bar"},
+			expectedSelector: "foo=bar",
+		},
+		{
+			name:             "multiple labels",
+			labels:           map[string]string{"foo": "bar", "foz": "baz"},
+			expectedSelector: "foo=bar,foz=baz",
+		},
+		{
+			name:             "nil map",
+			labels:           nil,
+			expectedSelector: "",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := Selector(tt.labels); got != tt.expectedSelector {
+				t.Errorf("Selector() = %v, want %v", got, tt.expectedSelector)
+			}
+		})
+	}
+}