util/syspolicy: add ReadStringArray interface (#11857)

Fixes tailscale/corp#19459

This PR adds the ability for users of the syspolicy handler to read string arrays from the MDM solution configured on the system.

Signed-off-by: Andrea Gottardo <andrea@gottardo.me>

util/syspolicy/syspolicy_test.go

@@ -18,6 +18,7 @@ type testHandler struct {
 	s     string
 	u64   uint64
 	b     bool
+	sArr  []string
 	err   error
 	calls int // used for testing reads from cache vs. handler
 }
@@ -48,6 +49,14 @@ func (th *testHandler) ReadBoolean(key string) (bool, error) {
 	return th.b, th.err
 }
 
+func (th *testHandler) ReadStringArray(key string) ([]string, error) {
+	if key != string(th.key) {
+		th.t.Errorf("ReadStringArray(%q) want %q", key, th.key)
+	}
+	th.calls++
+	return th.sArr, th.err
+}
+
 func TestGetString(t *testing.T) {
 	tests := []struct {
 		name         string