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/files.go

@@ -0,0 +1,38 @@
+// 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"
+	"os"
+)
+
+// writeHash writes a hash to file.
+func writeHash(fname, name string, hash []byte) error {
+	f, err := os.Create(fname)
+	if err != nil {
+		return err
+	}
+	fmt.Fprintf(f, "%x %s\n", hash, name)
+	return f.Close()
+}
+
+// writeFileHashes writes a file and its hashes to files.
+func writeFileHashes(fname, name string, data, s256, s512 []byte) error {
+	// Write the file itself.
+	if err := os.WriteFile(fname, data, 0644); err != nil {
+		return err
+	}
+	// Write SHA256 sum.
+	if err := writeHash(fname+".sha256", name, s256); err != nil {
+		return err
+	}
+	// Write SHA512 sum.
+	return writeHash(fname+".sha512", name, s512)
+}