Downloader: Add forwarding to HTTP endpoint (#442)

* started with forwarding support in downloader

* Add missing files.

* Add missing files.

* Raise needed Go version

* More Go version bumping.

* Fix forwarding

* Go 1.21+ needed

* Make terminating forwarder more robust.

* Better var naming

* Remove dead code. Improve commentary.

* Prepare validation status adjustment.

* Move validations to functions to make them executable in a loop.

* Introduce validation mode flag (strict, unsafe)

cmd/csaf_downloader/main.go

@@ -30,6 +30,13 @@ func run(cfg *config, domains []string) error {
 	ctx, stop := signal.NotifyContext(ctx, os.Interrupt)
 	defer stop()
 
+	if cfg.ForwardURL != "" {
+		f := newForwarder(cfg)
+		go f.run()
+		defer f.close()
+		d.forwarder = f
+	}
+
 	return d.run(ctx, domains)
 }