add Upload URL to release API (#26663)

- Resolves https://codeberg.org/forgejo/forgejo/issues/580
- Return a `upload_field` to any release API response, which points to
the API URL for uploading new assets.
- Adds unit test.
- Adds integration testing to verify URL is returned correctly and that
upload endpoint actually works

---------

Co-authored-by: Gusted <postmaster@gusted.xyz>

templates/swagger/v1_json.tmpl

@@ -21090,6 +21090,10 @@
           "type": "string",
           "x-go-name": "Target"
         },
+        "upload_url": {
+          "type": "string",
+          "x-go-name": "UploadURL"
+        },
         "url": {
           "type": "string",
           "x-go-name": "URL"