Add top-level artifacts: section

[aguschin]

This PR adds a top-level declarative section that is going to be used instead of GTO's artifacts.yaml
Example:

# dvc.yaml
artifacts:
  myart:
    type: model
  hello:
    type: file
    path: hello.txt

[codecov]

Codecov Report

Patch coverage: 95.60% and project coverage change: -0.01 ⚠️

Comparison is base (8be71be) 92.94% compared to head (be5101e) 92.93%.

❗ Current head be5101e differs from pull request most recent head a4180a9. Consider uploading reports for the commit a4180a9 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9220      +/-   ##
==========================================
- Coverage   92.94%   92.93%   -0.01%     
==========================================
  Files         459      461       +2     
  Lines       37112    37202      +90     
  Branches     5342     5357      +15     
==========================================
+ Hits        34493    34575      +82     
- Misses       2089     2093       +4     
- Partials      530      534       +4     

Impacted Files	Coverage Δ
dvc/repo/artifacts.py	88.23% <88.23%> (ø)
dvc/annotations.py	100.00% <100.00%> (ø)
dvc/dvcfile.py	96.76% <100.00%> (+0.04%)	⬆️
dvc/repo/__init__.py	94.75% <100.00%> (+0.02%)	⬆️
dvc/repo/index.py	92.75% <100.00%> (+0.06%)	⬆️
dvc/schema.py	100.00% <100.00%> (ø)
tests/func/artifacts/test_artifacts.py	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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[skshetry]

I'd have preferred not to use FileMixin, it's too bloated. But I am fine for now.

[skshetry]

Looks good to me so far.

[dberenbaum]

@aguschin Let's update it to handle paths like top-level plots (use path if provided, otherwise assume ID is a path). Once you have that ready and any other cleanup, you can ping the reviewers here for a final review. Thanks!

[aguschin]

Ok, ready for another review if needed :)
@dberenbaum, @skshetry @daavoo

[dberenbaum]

Thanks @aguschin! Could you also add monorepo tests per iterative/gto#198 (comment)?