Add Gitee importer (Gitee import 3/4)

What does this MR do and why?

Related to issue https://jihulab.com/gitlab-cn/gitlab/-/issues/1521

1. Add feature flag to control whether enable Gitee import.
2. Add Gitee importer.
3. Add import_sources.

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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