config: add yaml flag

YAML flag parses (preferably flow-style) YAML value from the command line
or unmarshals object yaml value from the yaml config file.

Example value:
```
bin/skipper -foo-flag='{foo: hello, bar: [world, "1"], baz: 2, qux: {baz: 3}}'
```

and equivalent branch in config yaml:
```yaml
foo-flag:
  foo: hello
  bar:
    - world
    - "1"
  baz: 2
  qux:
    baz: 3
```

This will be useful for #2104

It is also a better alternative to manual parsing of micro-syntax like
e.g. implemented in #2888

Signed-off-by: Alexander Yastrebov <alexander.yastrebov@zalando.de>

config/yamlflag.go

@@ -0,0 +1,42 @@
+package config
+
+import (
+	"fmt"
+
+	"gopkg.in/yaml.v2"
+)
+
+type yamlFlag[T any] struct {
+	Ptr   **T
+	value string // only for Set
+}
+
+func newYamlFlag[T any](ptr **T) *yamlFlag[T] {
+	return &yamlFlag[T]{Ptr: ptr}
+}
+
+func (yf *yamlFlag[T]) Set(value string) error {
+	var opts T
+	if err := yaml.Unmarshal([]byte(value), &opts); err != nil {
+		return fmt.Errorf("failed to parse yaml: %w", err)
+	}
+	*yf.Ptr = &opts
+	yf.value = value
+	return nil
+}
+
+func (yf *yamlFlag[T]) UnmarshalYAML(unmarshal func(interface{}) error) error {
+	var opts T
+	if err := unmarshal(&opts); err != nil {
+		return err
+	}
+	*yf.Ptr = &opts
+	return nil
+}
+
+func (yf *yamlFlag[T]) String() string {
+	if yf == nil {
+		return ""
+	}
+	return yf.value
+}

config/yamlflag_test.go

@@ -0,0 +1,83 @@
+package config
+
+import (
+	"testing"
+
+	"gopkg.in/yaml.v2"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+type yamlFlagTestConfig struct {
+	Foo string
+	Bar []string
+	Baz int
+	Qux *yamlFlagTestConfig
+}
+
+func TestYamlFlag(t *testing.T) {
+	t.Run("set", func(t *testing.T) {
+		cfg := struct {
+			Field *yamlFlagTestConfig
+		}{}
+
+		f := newYamlFlag(&cfg.Field)
+		v := `{foo: hello, bar: [world, "1"], baz: 2, qux: {baz: 3}}`
+		f.Set(v)
+
+		assert.Equal(t, "hello", cfg.Field.Foo)
+		assert.Equal(t, []string{"world", "1"}, cfg.Field.Bar)
+		assert.Equal(t, 2, cfg.Field.Baz)
+		assert.Equal(t, 3, cfg.Field.Qux.Baz)
+
+		assert.Equal(t, v, f.String())
+	})
+
+	t.Run("set empty", func(t *testing.T) {
+		cfg := struct {
+			Field *yamlFlagTestConfig
+		}{}
+
+		f := newYamlFlag(&cfg.Field)
+		f.Set("")
+
+		assert.Equal(t, &yamlFlagTestConfig{}, cfg.Field)
+		assert.Equal(t, "", f.String())
+	})
+
+	t.Run("set empty object", func(t *testing.T) {
+		cfg := struct {
+			Field *yamlFlagTestConfig
+		}{}
+
+		f := newYamlFlag(&cfg.Field)
+		f.Set("{}")
+
+		assert.Equal(t, &yamlFlagTestConfig{}, cfg.Field)
+		assert.Equal(t, "{}", f.String())
+	})
+
+	t.Run("unmarshal YAML", func(t *testing.T) {
+		cfg := struct {
+			Field *yamlFlagTestConfig
+		}{}
+
+		err := yaml.Unmarshal([]byte(`
+field:
+  foo: hello
+  bar:
+    - world
+    - "1"
+  baz: 2
+  qux:
+    baz: 3
+`), &cfg)
+		require.NoError(t, err)
+
+		assert.Equal(t, "hello", cfg.Field.Foo)
+		assert.Equal(t, []string{"world", "1"}, cfg.Field.Bar)
+		assert.Equal(t, 2, cfg.Field.Baz)
+		assert.Equal(t, 3, cfg.Field.Qux.Baz)
+	})
+}