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/client.go

@@ -0,0 +1,72 @@
+// 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 (
+	"context"
+	"errors"
+	"io"
+	"net/http"
+	"net/url"
+
+	"golang.org/x/time/rate"
+)
+
+type client interface {
+	Do(req *http.Request) (*http.Response, error)
+	Get(url string) (*http.Response, error)
+	Head(url string) (*http.Response, error)
+	Post(url, contentType string, body io.Reader) (*http.Response, error)
+	PostForm(url string, data url.Values) (*http.Response, error)
+}
+
+type limitingClient struct {
+	client
+	limiter *rate.Limiter
+}
+
+func (lc *limitingClient) Do(req *http.Request) (*http.Response, error) {
+	lc.limiter.Wait(context.Background())
+	return lc.client.Do(req)
+}
+
+func (lc *limitingClient) Get(url string) (*http.Response, error) {
+	lc.limiter.Wait(context.Background())
+	return lc.client.Get(url)
+}
+
+func (lc *limitingClient) Head(url string) (*http.Response, error) {
+	lc.limiter.Wait(context.Background())
+	return lc.client.Head(url)
+}
+
+func (lc *limitingClient) Post(url, contentType string, body io.Reader) (*http.Response, error) {
+	lc.limiter.Wait(context.Background())
+	return lc.client.Post(url, contentType, body)
+}
+
+func (lc *limitingClient) PostForm(url string, data url.Values) (*http.Response, error) {
+	lc.limiter.Wait(context.Background())
+	return lc.client.PostForm(url, data)
+}
+
+var errNotFound = errors.New("not found")
+
+func downloadJSON(c client, url string, found func(io.Reader) error) error {
+	res, err := c.Get(url)
+	if err != nil || res.StatusCode != http.StatusOK ||
+		res.Header.Get("Content-Type") != "application/json" {
+		// ignore this as it is expected.
+		return errNotFound
+	}
+	return func() error {
+		defer res.Body.Close()
+		return found(res.Body)
+	}()
+}