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>

services/convert/release.go

@@ -22,6 +22,7 @@ func ToAPIRelease(ctx context.Context, repo *repo_model.Repository, r *repo_mode
 		HTMLURL:      r.HTMLURL(),
 		TarURL:       r.TarURL(),
 		ZipURL:       r.ZipURL(),
+		UploadURL:    r.APIUploadURL(),
 		IsDraft:      r.IsDraft,
 		IsPrerelease: r.IsPrerelease,
 		CreatedAt:    r.CreatedUnix.AsTime(),