Add aggregator; improve itest workflow

* Factor JSON evaluation and  construction base URLs out of of checker.
* Move json path matching to util.
* Add csaf_aggregator (as additional command)
* Improve itest workflow to checkout the branch where it is running on.

resolve #105
resolve  #72

Co-authored-by: tschmidtb51 <65305130+tschmidtb51@users.noreply.github.com>
Co-authored-by: Bernhard Reiter <bernhard@intevation.de>
Co-authored-by: Fadi Abbud <fadi.abbud@intevation.de>

cmd/csaf_aggregator/main.go

@@ -0,0 +1,76 @@
+// 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/gofrs/flock"
+	"github.com/jessevdk/go-flags"
+)
+
+type options struct {
+	Config  string `short:"c" long:"config" description:"File name of the configuration file" value-name:"CFG-FILE" default:"aggregator.toml"`
+	Version bool   `long:"version" description:"Display version of the binary"`
+	Interim bool   `short:"i" long:"interim" description:"Perform an interim scan"`
+}
+
+func errCheck(err error) {
+	if err != nil {
+		if e, ok := err.(*flags.Error); ok && e.Type == flags.ErrHelp {
+			os.Exit(0)
+		}
+		log.Fatalf("error: %v\n", err)
+	}
+}
+
+func lock(lockFile *string, fn func() error) error {
+	if lockFile == nil {
+		// No locking configured.
+		return fn()
+	}
+	fl := flock.New(*lockFile)
+	locked, err := fl.TryLock()
+	if err != nil {
+		return fmt.Errorf("file locking failed: %v", err)
+	}
+
+	if !locked {
+		return fmt.Errorf("cannot lock to file %s", *lockFile)
+	}
+	defer fl.Unlock()
+	return fn()
+}
+
+func main() {
+	opts := new(options)
+
+	_, err := flags.Parse(opts)
+	errCheck(err)
+
+	if opts.Version {
+		fmt.Println(util.SemVersion)
+		return
+	}
+
+	interim := opts.Interim
+
+	cfg, err := loadConfig(opts.Config)
+	errCheck(err)
+
+	if interim {
+		cfg.Interim = true
+	}
+
+	p := processor{cfg: cfg}
+	errCheck(lock(cfg.LockFile, p.process))
+}