feat: implement imprint/impressum support (#706)

* feat: implement imprint/impressum support

Closes #362

Signed-off-by: Xe Iaso <me@xeiaso.net>

* chore(docs/anubis): enable an imprint

Signed-off-by: Xe Iaso <me@xeiaso.net>

* chore: spelling

Signed-off-by: Xe Iaso <me@xeiaso.net>

* docs: fix the end of the sentence, comment out a default impressum

Signed-off-by: Xe Iaso <me@xeiaso.net>

* docs: link back to impressum page

Signed-off-by: Xe Iaso <me@xeiaso.net>

---------

Signed-off-by: Xe Iaso <me@xeiaso.net>

lib/policy/config/impressum.go

@@ -0,0 +1,71 @@
+package config
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+)
+
+var ErrMissingValue = errors.New("config: missing value")
+
+type Impressum struct {
+	Footer string        `json:"footer" yaml:"footer"`
+	Page   ImpressumPage `json:"page" yaml:"page"`
+}
+
+func (i Impressum) Render(_ context.Context, w io.Writer) error {
+	if _, err := fmt.Fprint(w, i.Footer); err != nil {
+		return err
+	}
+	return nil
+}
+
+func (i Impressum) Valid() error {
+	var errs []error
+
+	if len(i.Footer) == 0 {
+		errs = append(errs, fmt.Errorf("%w: impressum footer must be defined", ErrMissingValue))
+	}
+
+	if err := i.Page.Valid(); err != nil {
+		errs = append(errs, err)
+	}
+
+	if len(errs) != 0 {
+		return errors.Join(errs...)
+	}
+
+	return nil
+}
+
+type ImpressumPage struct {
+	Title string `json:"title" yaml:"title"`
+	Body  string `json:"body" yaml:"body"`
+}
+
+func (ip ImpressumPage) Render(_ context.Context, w io.Writer) error {
+	if _, err := fmt.Fprint(w, ip.Body); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func (ip ImpressumPage) Valid() error {
+	var errs []error
+
+	if len(ip.Title) == 0 {
+		errs = append(errs, fmt.Errorf("%w: impressum page title must be defined", ErrMissingValue))
+	}
+
+	if len(ip.Body) == 0 {
+		errs = append(errs, fmt.Errorf("%w: impressum body title must be defined", ErrMissingValue))
+	}
+
+	if len(errs) != 0 {
+		return errors.Join(errs...)
+	}
+
+	return nil
+}