experiments: checkpoints proof of concept

[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 close #4498.

• dvc exp run --checkpoint can be used to reproduce a checkpoint experiment
  • Checkpoint stages should be generated using --always-changed and with --outs-persist for the intermediate checkpoint outputs
  • When running a checkpoint experiment, a new commit in the experiment branch will be generated each time the running stage calls dvc.api.make_checkpoint() (or generates the appropriate .dvc/tmp/DVC_CHECKPOINT) signal file
  • Reproduction and checkpoint generation will run forever until the user code exits or is killed via Ctrl+C
• dvc exp run --continue <checkpoint_exp_rev> can be used to resume a prior checkpoint experiment. Execution will be resumed from the tip of the checkpoint branch.
• Checkpoint experiments can be viewed as normal in dvc exp show

Known issues (needs further investigation after this PR):

• Git-python sometimes throws broken pipe errors when checkpoint runs are killed via Ctrl-C
• Reproducing a baseline commit does not work properly, for a checkpoint experiment to be generated properly, there needs to be some change versus the baseline/parent commit (either in the workspace or via the repro --params option)
• Sorting in dvc exp show will not work properly if the table contains checkpoint experiments, but filtering should work as expected

Features that will need follow up PR:

• Branching from some commit in the middle of a checkpoint experiment and then resuming a "new" branch with potentially modified code/params is not yet supported

[pmrowla]

all experiments run/repro behavior will be moved from dvc repro to dvc exp run in a follow-up PR, for now repro -e is just duplicated here so that checkpoint runs work correctly

[pmrowla]

[efiop]

Great start!