Add checkpoint backup

[karajan1001]

Per iterative/dvc#6332

[karajan1001]

The core PR had been merged, we can continue on this one. For now we have two envs:

1. DVC_EXP_GIT_REMOTE: Remote Git repo for the exp refs to back up. Can be a URL or a name ( origin, myremote, etc). This can be used for default git remote in dvc exp push
2. DVC_EXP_AUTO_PUSH: trigger of the auto push. (true, 1, y, yes) for Ture.

Other things to be noticed:

1. There is a validation in the beginning of the iteration, if one of DVC_EXP_GIT_REMOTE and dvc default remote cache is not setted , it will raise exception and error out.
2. But if the validation passed and the auto push fails during the iterations it will only give a warning and continue the process.
3. The DVC_EXP_GIT_REMOTE can be set to the running repo. In this case cache will be pushed to dvc remote and exp refs will not (it is already in the running repo).

What else is needed? Some examples or other things?

[karajan1001]

Not good at English expression need some help.

[iesahin]

谢谢 @karajan1001 Taking this over.

[iesahin]

Ah, excuse me, @casperdcl already assigned this to himself. Thank you :)

[casperdcl]

reworded; keeping header for #id backlink user convenience/referencing.

[iesahin]

I think we can split this into two warnings.

[jorgeorpinel]

What are the 2 warnings?

[jorgeorpinel]

Where is Resyled when you need it 😢 it's been acting up recently...