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>

docs/examples/aggregator.toml

@@ -0,0 +1,33 @@
+workers = 2
+folder = "/var/csaf-aggregator"
+web = "/var/csaf-aggregator/html"
+domain = "https://localhost:9443"
+rate = 10.0
+insecure = true
+[aggregator]
+  category = "aggregator"
+  name = "Example Development CSAF Aggregator"
+  contact_details = "some @ somewhere"
+  issuing_authority = "This service is provided as it is. It is gratis for everybody."
+  namespace = "Testnamespace"
+
+[[providers]]
+  name = "local-dev-provider"
+  domain = "localhost"
+#  rate = 1.5
+#  insecure = true
+
+[[providers]]
+  name = "local-dev-provider2"
+  domain = "localhost"
+#  rate = 1.2
+#  insecure = true
+
+#key =
+#passphrase =
+
+# for testing, the specifiation requires at least two
+# allow_single_provider = true
+
+
+