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/sources/source.go

@@ -0,0 +1,8 @@
+package sources
+
+import "github.com/qdm12/gluetun/internal/configuration/settings"
+
+type Source interface {
+	Read() (settings settings.Settings, err error)
+	ReadHealth() (settings settings.Health, err error)
+}