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/mux/reader.go

@@ -0,0 +1,57 @@
+package mux
+
+import (
+	"fmt"
+
+	"github.com/qdm12/gluetun/internal/configuration/settings"
+	"github.com/qdm12/gluetun/internal/configuration/sources"
+)
+
+var _ sources.Source = (*Reader)(nil)
+
+type Reader struct {
+	sources []sources.Source
+}
+
+func New(sources ...sources.Source) *Reader {
+	return &Reader{
+		sources: sources,
+	}
+}
+
+// Read reads the settings for each source, merging unset fields
+// with field set by the next source.
+// It then set defaults to remaining unset fields.
+func (r *Reader) Read() (settings settings.Settings, err error) {
+	for _, source := range r.sources {
+		settingsFromSource, err := source.Read()
+		if err != nil {
+			return settings, fmt.Errorf("cannot read from source %T: %w", source, err)
+		}
+		settings.MergeWith(settingsFromSource)
+	}
+	settings.SetDefaults()
+	return settings, nil
+}
+
+// ReadHealth reads the health settings for each source, merging unset fields
+// with field set by the next source.
+// It then set defaults to remaining unset fields, and validate
+// all the fields.
+func (r *Reader) ReadHealth() (settings settings.Health, err error) {
+	for _, source := range r.sources {
+		settingsFromSource, err := source.ReadHealth()
+		if err != nil {
+			return settings, fmt.Errorf("cannot read from source %T: %w", source, err)
+		}
+		settings.MergeWith(settingsFromSource)
+	}
+	settings.SetDefaults()
+
+	err = settings.Validate()
+	if err != nil {
+		return settings, err
+	}
+
+	return settings, nil
+}