Add Average Precision to metrics

[thibaultdvx]

Fixes #8085.

Description

Average Precision is very similar to ROCAUC, so I was very much inspired by the ROCAUC implementation.
More precisely, I created:

• AveragePrecisionMetric and compute_average_precision in monai.metrics,
• a handler called AveragePrecision in monai.handlers,
• three unittest modules: test_compute_average_precision.py, test_handler_average_precision.py and test_handler_average_precision_dist.py.

I also modified the docs to mention Average Precision.

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.