๐ŸŒ… add fetch_repo subcommand (based on fetchsrclibs plugin)

We've been using fetchsrclibs plugin for downloading source-code for
apps and srclibs. This change copies to plugin script into fdroidserver
as a subcommand. It also renames it from fetchsrclibs to fetch_repo
because it's fetching the git repo in Repo:
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f