git clone authentication

[dtrifiro]

    Do you think we should specify that DVC does not support interactive auth, and so credentials must be saved somewhere and automatically available when running Git commands to access a private repo?

Originally posted by @dberenbaum in #4211 (comment)

[dberenbaum]

I think we should make this a broader troubleshooting to explain common issues users might face when DVC needs to authenticate a Git repo. Some example issues that could be covered (I think there have been more):

• DVCFileSystem cannot be instantiated twice for a repo with http auth dvc#9012
• ssh issues with encrypted identity files on MacOS dvc-ssh#20

Things to include:

• DVC will read from credential helpers; suggest to use those.
• DVC does not support interactive authentication, so the credentials must be stored somewhere already (like from having git cloned separately) before you can use them.
• Keep in mind that local/repo-specific config will not be used when cloning another repo.
• Mention known limitations like ssh issues with encrypted identity files on MacOS dvc-ssh#20.

[dberenbaum]

We also claim that exp push/pull only support SSH auth, which I don't think should be true anymore.