[Feat] Add KeypointAUC metric

[LareinaM]

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Motivation

Add new metric KeypointAUC

Modification

1. Add mmeval/metrics/keypoint_auc.py
2. Modify metric doc and __init__.py
3. Add unit test tests/test_metrics/test_keypoint_auc.py

Related PR on MMPose

open-mmlab/mmpose#1985
Update AUC to use MMEval

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[C1rN09]

Approved by mistake

[C1rN09]

LGTM