experiments: store experiments in refs/exps instead of heads

[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. 🙏

Related to #4897.

• Experiments no longer uses separate .dvc/experiments clone/workspace
• Experiments are now stored under a custom git refspec namespace: refs/exps/...
  • Queued experiments are stored in a separate stash: refs/exps/stash
• Experiment executors are now populated and collected by pushing/fetching git refs
• dvc exp checkout has been replaced with dvc exp apply
  • Experiment results are no longer applied to the user's workspace by default
  • dvc exp apply will apply the changes from an experiment to the workspace (as unstaged changes)
• Experiments can be named at runtime using dvc exp run -n/--name <name> ... (also applies to resume w/modified branch params)
• dvc exp branch <experiment> <branch_name> can be used to promote an experiment to a normal (refs/heads/...) Git branch
• All other existing behavior remains unchanged from the UI perspective

TODO:

• dvc exp branch will "promote" an experiment branch to traditional (refs/heads) git branch
• Support human-readable experiment ref names

[jorgeorpinel]

@pmrowla I had an idea: since your PR descriptions have pretty complete explanations of what's being done, can we list them all somewhere (perhaps in https://github.com/iterative/dvc/wiki/Experiments) as a first step towards QA and docs this month? Thanks!

[pmrowla]

@jorgeorpinel As far as anything user-facing goes, everything from these PRs is already on the wiki page.

As far as viewing all the PRs goes, you can just filter by the experiments label:
https://github.com/iterative/dvc/pulls?q=is%3Apr+label%3Aexperiments+