checkpoints: flag to `exp push` && `push`

[casperdcl]

Provide a flag (e.g. dvclive.next_step(push=True) or make_checkpoint(push=True) or DVC_EXP_AUTO_PUSH=<git_remote_name_or_url|true>) to push things per-checkpoint.

Helps with resuming a long-running CI job after timeout.

Part of iterative/cml#560.

[karajan1001]

@efiop, Some analysis after looking into it.

1. This is a DVC only option. Shouldn't do any change in dvclive which would make it rely on DVC.
2. It would be running after every turn of the stage running,  we can create a new monitor task for it like other dvclive ones for it.
3. A new parameter in the stage file is needed to control this process.

Any suggestions for this?

[dberenbaum]

Can this be an argument to make_checkpoint()?

[pmrowla]

Given that this is a CI thing, it seems like maybe it should just be set through an env var?

something like like DVC_EXP_AUTO_PUSH=<git_remote_name_or_url>

[pmrowla]

Also note that for CML purposes, I think we will want both DVC cache and run-cache to be included with the automated exp push (DVC cache is already included by default on exp push but run cache is normally optional)