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/issue_a.json

@@ -0,0 +1,14 @@
+[
+  {
+      "number": 1,
+      "poster_id": 1,
+      "poster_name": "name_a",
+      "title": "title_a",
+      "content": "content_a",
+      "state": "closed",
+      "is_locked": false,
+      "created": "1985-04-12T23:20:50.52Z",
+      "updated": "1986-04-12T23:20:50.52Z",
+      "closed": "1987-04-12T23:20:50.52Z"
+  }
+]