Support syncing assets for other repos

[blongv]

As mentioned in actions-sync issue 43, we need a tool to sync not only actions git repos but also assets (release binaries) from github.com to Github Enterprise Servers. This is especially true for repos like actions/node-versions. Adding the capability for mirroring node-versions and other frequently-referenced assets would be instrumental in reducing traffic to github.com and also avoid rate limits for users of Github Enterprise Server.