Allow instantiating template dataclasses in jinja2 templates

In some cases, the template data might need to be set directly in the
jinja2 templates using `{% set %}`. Expose the template dataclasses to
the templates so we can use these existing classes, rather than creating
ad-hoc data containers.

bin/tests/system/isctest/template.py

@@ -45,6 +45,11 @@ class TemplateEngine:
             variable_start_string="@",
             variable_end_string="@",
         )
+        # allow instantiating the template dataclasses in jinja2 templates when
+        # using {% set %}
+        self.j2env.globals["Nameserver"] = Nameserver
+        self.j2env.globals["TrustAnchor"] = TrustAnchor
+        self.j2env.globals["Zone"] = Zone
 
     def render(
         self,