Validate migration files (#18203)

JSON Schema validation for data used by Gitea during migrations

Discussion at https://forum.forgefriends.org/t/common-json-schema-for-repository-information/563

Co-authored-by: Loïc Dachary <loic@dachary.org>

modules/migration/file_format_testdata/milestones.json

@@ -0,0 +1,20 @@
+[
+  {
+      "title": "title_a",
+      "description": "description_a",
+      "deadline": "1988-04-12T23:20:50.52Z",
+      "created": "1985-04-12T23:20:50.52Z",
+      "updated": "1986-04-12T23:20:50.52Z",
+      "closed": "1987-04-12T23:20:50.52Z",
+      "state": "closed"
+  },
+  {
+      "title": "title_b",
+      "description": "description_b",
+      "deadline": "1998-04-12T23:20:50.52Z",
+      "created": "1995-04-12T23:20:50.52Z",
+      "updated": "1996-04-12T23:20:50.52Z",
+      "closed": null,
+      "state": "open"
+  }
+]