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>

csaf/util.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 csaf
+
+import (
+	"bufio"
+	"io"
+	"strings"
+)
+
+// ExtractProviderURL extracts URLs of provider metadata.
+// If all is true all URLs are returned. Otherwise only the first is returned.
+func ExtractProviderURL(r io.Reader, all bool) ([]string, error) {
+	const csaf = "CSAF:"
+
+	var urls []string
+
+	sc := bufio.NewScanner(r)
+	for sc.Scan() {
+		line := sc.Text()
+		if strings.HasPrefix(line, csaf) {
+			urls = append(urls, strings.TrimSpace(line[len(csaf):]))
+			if !all {
+				return urls, nil
+			}
+		}
+	}
+	if err := sc.Err(); err != nil {
+		return nil, err
+	}
+	return urls, nil
+}