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/settings_test.go

@@ -0,0 +1,101 @@
+package settings
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_Settings_String(t *testing.T) {
+	t.Parallel()
+
+	withDefaults := func(s Settings) Settings {
+		s.SetDefaults()
+		return s
+	}
+
+	testCases := map[string]struct {
+		settings Settings
+		s        string
+	}{
+		"default settings": {
+			settings: withDefaults(Settings{}),
+			s: `Settings summary:
+├── VPN settings:
+|   ├── VPN provider settings:
+|   |   ├── Name: private internet access
+|   |   └── Server selection settings:
+|   |       ├── VPN type: openvpn
+|   |       └── OpenVPN server selection settings:
+|   |           ├── Protocol: UDP
+|   |           └── Private Internet Access encryption preset: strong
+|   └── OpenVPN server selection settings:
+|       ├── OpenVPN version: 2.5
+|       ├── User: [not set]
+|       ├── Password: [not set]
+|       ├── Private Internet Access encryption preset: strong
+|       ├── Tunnel IPv6: no
+|       ├── Network interface: tun0
+|       ├── Run OpenVPN as: root
+|       └── Verbosity level: 1
+├── DNS settings:
+|   ├── DNS server address to use: 127.0.0.1
+|   ├── Keep existing nameserver(s): no
+|   └── DNS over TLS settings:
+|       ├── Enabled: yes
+|       ├── Update period: every 24h0m0s
+|       ├── Unbound settings:
+|       |   ├── Authoritative servers:
+|       |   |   └── Cloudflare
+|       |   ├── Caching: yes
+|       |   ├── IPv6: no
+|       |   ├── Verbosity level: 1
+|       |   ├── Verbosity details level: 0
+|       |   ├── Validation log level: 0
+|       |   ├── System user: root
+|       |   └── Allowed networks:
+|       |       ├── 0.0.0.0/0
+|       |       └── ::/0
+|       └── DNS filtering settings:
+|           ├── Block malicious: yes
+|           ├── Block ads: no
+|           └── Block surveillance: yes
+├── Firewall settings:
+|   └── Enabled: yes
+├── Log settings:
+|   └── Log level: INFO
+├── Health settings:
+|   ├── Server listening address: 127.0.0.1:9999
+|   ├── Address to ping: github.com
+|   └── VPN wait durations:
+|       ├── Initial duration: 6s
+|       └── Additional duration: 5s
+├── Shadowsocks server settings:
+|   └── Enabled: no
+├── HTTP proxy settings:
+|   └── Enabled: no
+├── Control server settings:
+|   ├── Listening port: 8000
+|   └── Logging: yes
+├── OS Alpine settings:
+|   ├── Process UID: 1000
+|   └── Process GID: 1000
+├── Public IP settings:
+|   ├── Fetching: every 12h0m0s
+|   └── IP file path: /tmp/gluetun/ip
+└── Version settings:
+    └── Enabled: yes`,
+		},
+	}
+
+	for name, testCase := range testCases {
+		testCase := testCase
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			s := testCase.settings.String()
+
+			assert.Equal(t, testCase.s, s)
+		})
+	}
+}