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/unbound_test.go

@@ -0,0 +1,43 @@
+package settings
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"inet.af/netaddr"
+)
+
+func Test_Unbound_JSON(t *testing.T) {
+	t.Parallel()
+
+	settings := Unbound{
+		Providers:             []string{"cloudflare"},
+		Caching:               boolPtr(true),
+		IPv6:                  boolPtr(false),
+		VerbosityLevel:        uint8Ptr(1),
+		VerbosityDetailsLevel: nil,
+		ValidationLogLevel:    uint8Ptr(0),
+		Username:              "user",
+		Allowed: []netaddr.IPPrefix{
+			netaddr.IPPrefixFrom(netaddr.IPv4(0, 0, 0, 0), 0),
+			netaddr.IPPrefixFrom(netaddr.IPv6Raw([16]byte{}), 0),
+		},
+	}
+
+	b, err := json.Marshal(settings)
+	require.NoError(t, err)
+
+	const expected = `{"Providers":["cloudflare"],"Caching":true,"IPv6":false,` +
+		`"VerbosityLevel":1,"VerbosityDetailsLevel":null,"ValidationLogLevel":0,` +
+		`"Username":"user","Allowed":["0.0.0.0/0","::/0"]}`
+
+	assert.Equal(t, expected, string(b))
+
+	var resultSettings Unbound
+	err = json.Unmarshal(b, &resultSettings)
+	require.NoError(t, err)
+
+	assert.Equal(t, settings, resultSettings)
+}