api: Repo.stage for loading/dumping stages

[skshetry]

Right now, this service was provided through Dvcfile.stages and Dvcfile.stage. The problem with the API is it requires us to create Dvcfile each time even if we don't need it.

We already have wrappers around it due to above reason:

dvc/dvc/repo/__init__.py

Line 258 in 3daeeaf

def get_stage(self, path=None, name=None):

dvc/dvc/repo/__init__.py

Line 266 in 3daeeaf

def get_stages(self, path=None, name=None):

This might get more complicated with group stage run target, i.e. name of the stage group of a foreach.
So, it'd be great to have a simple wrapper that'd look like this:

>>> repo.stage.load_one("build", file="dvc.yaml")
>>> repo.stage.load_all(file="data.csv.dvc")
>>> repo.stage.load(target="", file="")

The API might improve, when trying to improve this. That is what I have in mind.

[efiop]

@skshetry Is it required for some other feature/usecase? Just wondering about the background.