exp push/pull/list: support git credential storage

[dberenbaum]

Extracted from #6493.

dvc exp push/pull/list do not work today if credentials are required to be provided, such as dvc exp push to a public Github HTTP URL or dvc exp push/pull/list for a private Github HTTP URL.

Also, even if using git credential stores, in which case users don't need to manually enter credentials when performing git operations, DVC doesn't currently read these credentials and still complains that the URL is unauthenticated. Note that dulwich seems to have support for this: https://github.com/dulwich/dulwich/blob/cdc93e058fb28e4884f8d6a23b4d79b14d2a8098/dulwich/client.py#L2222.

As discussed in the linked PR, it would be helpful to read from the credential store and ask for the username and password if the HTTPClientUnauthorized exception is thrown. It would also be helpful to memoize to avoid having to repeatedly ask for credentials in the case of auto-pushing checkpoints. This might be generally useful because obtaining a new git client for each checkpoint seems wasteful regardless.

[pmrowla]

this feature req applies to DVC in general (and not just exp sharing) now that clone has been completely migrated to using dulwich/pygit instead of gitpython

related: #6586 #7674

[pmrowla]

Adding this to the backlog as follow-up to the workaround in #7674

from #7674 (comment):

Ideally we would implement support for credential helpers in either dulwich (jelmer/dulwich#873) or libgit2/pygit (libgit2/libgit2#4873)

Adding support for this requires implementing the git credentials storage protocol in dulwich and/or libgit. On the dulwich side, essentially this just means:

• reading the name/path of an executable from the git config
• run the executable in subprocess with the appropriate action args (get/store/erase)
• read/write the key=value protocol values to/from the process as described in the git docs

We also probably only need read-only support for now (i.e. likely only need to implement the get action)