experiments: support binary and untracked files in experiment patches

[pmrowla]

• ❗ I have followed the Contributing to DVC checklist.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

Will fix #4451

[pmrowla]

One concern here would be that we could potentially be sticking large binary files into git, which git is bad at handling and why dvc exists in the first place. (But they would only go into the experiments workspace git repo, and not the user's main repo)

I'm not sure if there's a cleaner way for us to handle this situation other than documenting that best practices here would be to either mark these types of large binary intermediate artifacts/outputs as dvc outs, or to gitignore them.

[pmrowla]

As a potential future improvement, we could maybe consider doing something like generating a directory containing all of the untracked files generated by an experiment run (along with a manifest or something containing the original relpaths for the untracked files), and then storing that as an experiment-specific dvc-tracked directory. But in the meantime, this bugfix is needed for testing/development purposes.