chore!(amneziawg): refactor to be separate from wireguard

- amneziawg is now a VPN protocol and no longer a Wireguard implementation
- Use it with VPN_TYPE=amneziawg
- document AMNEZIAWG_* options in Dockerfile
- document amneziawg support in readme
- separate amneziawg settings and code from wireguard
- re-use code from wireguard whenever possible

internal/configuration/sources/secrets/amneziawg.go

@@ -0,0 +1,27 @@
+package secrets
+
+import (
+	"os"
+	"path/filepath"
+
+	"github.com/qdm12/gluetun/internal/configuration/sources/files"
+)
+
+func (s *Source) lazyLoadAmneziawgConf() files.AmneziawgConfig {
+	if s.cached.amneziawgLoaded {
+		return s.cached.amneziawgConf
+	}
+
+	path := os.Getenv("AMNEZIAWG_CONF_SECRETFILE")
+	if path == "" {
+		path = filepath.Join(s.rootDirectory, "amneziawg", "awg0.conf")
+	}
+
+	s.cached.amneziawgLoaded = true
+	var err error
+	s.cached.amneziawgConf, err = files.ParseAmneziawgConf(path)
+	if err != nil {
+		s.warner.Warnf("skipping Amneziawg config: %s", err)
+	}
+	return s.cached.amneziawgConf
+}