[FEAT] add Forgejo Git Service (squash) register a Forgejo factory

If the Forgejo factory for the Forgejo service is not registered,
newDownloader will fallback to a git service and not migrate issues
etc.

Refs: https://codeberg.org/forgejo/forgejo/issues/1678
(cherry picked from commit 2ecb2558be696965a49b543d5e843526c105e07a)

Do not include tests so it is kept minimal.

services/migrations/forgejo_downloader.go

@@ -0,0 +1,20 @@
+// Copyright 2023 The Forgejo Authors
+// SPDX-License-Identifier: MIT
+
+package migrations
+
+import (
+	"code.gitea.io/gitea/modules/structs"
+)
+
+func init() {
+	RegisterDownloaderFactory(&ForgejoDownloaderFactory{})
+}
+
+type ForgejoDownloaderFactory struct {
+	GiteaDownloaderFactory
+}
+
+func (f *ForgejoDownloaderFactory) GitServiceType() structs.GitServiceType {
+	return structs.ForgejoService
+}