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/dns/state/state.go

@@ -3,7 +3,7 @@ package state
 import (
 	"sync"
 
-	"github.com/qdm12/gluetun/internal/configuration"
+	"github.com/qdm12/gluetun/internal/configuration/settings"
 	"github.com/qdm12/gluetun/internal/loopstate"
 )
 
@@ -14,7 +14,7 @@ type Manager interface {
 }
 
 func New(statusApplier loopstate.Applier,
-	settings configuration.DNS,
+	settings settings.DNS,
 	updateTicker chan<- struct{}) *State {
 	return &State{
 		statusApplier: statusApplier,
@@ -26,7 +26,7 @@ func New(statusApplier loopstate.Applier,
 type State struct {
 	statusApplier loopstate.Applier
 
-	settings   configuration.DNS
+	settings   settings.DNS
 	settingsMu sync.RWMutex
 
 	updateTicker chan<- struct{}