Add CSAF downloader

* Dense and refactor ROLIE code in aggregator a bit.
* Move  advisory file processor to csaf package.
* Fix minor typo on main readme

cmd/csaf_downloader/main.go

@@ -0,0 +1,59 @@
+// This file is Free Software under the MIT License
+// without warranty, see README.md and LICENSES/MIT.txt for details.
+//
+// SPDX-License-Identifier: MIT
+//
+// SPDX-FileCopyrightText: 2022 German Federal Office for Information Security (BSI) <https://www.bsi.bund.de>
+// Software-Engineering: 2022 Intevation GmbH <https://intevation.de>
+
+package main
+
+import (
+	"fmt"
+	"log"
+	"os"
+
+	"github.com/csaf-poc/csaf_distribution/util"
+	"github.com/jessevdk/go-flags"
+)
+
+type options struct {
+	Directory *string  `short:"d" long:"directory" description:"Directory to store the downloaded files in"`
+	Insecure  bool     `long:"insecure" description:"Do not check TLS certificates from provider"`
+	Version   bool     `long:"version" description:"Display version of the binary"`
+	Verbose   bool     `long:"verbose" short:"v" description:"Verbose output"`
+	Rate      *float64 `long:"rate" short:"r" description:"The average upper limit of https operations per second"`
+}
+
+func errCheck(err error) {
+	if err != nil {
+		if flags.WroteHelp(err) {
+			os.Exit(0)
+		}
+		log.Fatalf("error: %v\n", err)
+	}
+}
+
+func main() {
+
+	opts := new(options)
+
+	parser := flags.NewParser(opts, flags.Default)
+	parser.Usage = "[OPTIONS] domain..."
+	domains, err := parser.Parse()
+	errCheck(err)
+
+	if opts.Version {
+		fmt.Println(util.SemVersion)
+		return
+	}
+
+	if len(domains) == 0 {
+		log.Println("No domains given.")
+		return
+	}
+
+	d := downloader{opts: opts}
+
+	errCheck(d.run(domains))
+}