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/messages.go

@@ -0,0 +1,29 @@
+package helpers
+
+import (
+	"fmt"
+	"strings"
+)
+
+func ChoicesOrString(choices []string) string {
+	return strings.Join(
+		choices[:len(choices)-1], ", ") +
+		" or " + choices[len(choices)-1]
+}
+
+func PortChoicesOrString(ports []uint16) (s string) {
+	switch len(ports) {
+	case 0:
+		return "there is no allowed port"
+	case 1:
+		return "allowed port is " + fmt.Sprint(ports[0])
+	}
+
+	s = "allowed ports are "
+	portStrings := make([]string, len(ports))
+	for i := range ports {
+		portStrings[i] = fmt.Sprint(ports[i])
+	}
+	s += ChoicesOrString(portStrings)
+	return s
+}