feat(templates): carry content field requirements

tests/test_template_contract.py

@@ -15,6 +15,7 @@ from govoplan_core.core.templates import (
     TemplateCatalogProvider,
     TemplateContentDraftRequest,
     TemplateContentLibraryProvider,
+    TemplateFieldRequirement,
     TemplateRenderRequest,
     TemplateRendererProvider,
 )
@@ -89,8 +90,12 @@ class TemplateContractTests(unittest.TestCase):
             template_type="content_fragment",
             usages=("campaign.content",),
             content_text="Kind regards",
+            required_fields=(
+                TemplateFieldRequirement(path="local.display_name", label="Display name"),
+            ),
         )
         self.assertEqual("de", content.locale)
+        self.assertEqual("local.display_name", content.required_fields[0].path)
 
 
 if __name__ == "__main__":