chore(settings): refactor settings processing (#756)

- Better settings tree structure logged using `qdm12/gotree`
- Read settings from environment variables, then files, then secret files
- Settings methods to default them, merge them and override them
- `DNS_PLAINTEXT_ADDRESS` default changed to `127.0.0.1` to use DoT. Warning added if set to something else.
- `HTTPPROXY_LISTENING_ADDRESS` instead of `HTTPPROXY_PORT` (with retro-compatibility)

internal/configuration/settings/helpers/belong.go

@@ -0,0 +1,45 @@
+package helpers
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+)
+
+func IsOneOf(value string, choices ...string) (ok bool) {
+	for _, choice := range choices {
+		if value == choice {
+			return true
+		}
+	}
+	return false
+}
+
+var ErrValueNotOneOf = errors.New("value is not one of the possible choices")
+
+func AreAllOneOf(values, choices []string) (err error) {
+	set := make(map[string]struct{}, len(choices))
+	for _, choice := range choices {
+		choice = strings.ToLower(choice)
+		set[choice] = struct{}{}
+	}
+
+	for _, value := range values {
+		_, ok := set[value]
+		if !ok {
+			return fmt.Errorf("%w: value %q, choices available are %s",
+				ErrValueNotOneOf, value, strings.Join(choices, ", "))
+		}
+	}
+
+	return nil
+}
+
+func Uint16IsOneOf(port uint16, choices []uint16) (ok bool) {
+	for _, choice := range choices {
+		if port == choice {
+			return true
+		}
+	}
+	return false
+}