[WIP] MonaiAlgo FedStats

[holgerroth]

Fixes #5196.

Description

Add api function that allows FL systems to request summary statistics, i.e. get_data_stats in new ClientStats class. MonaiStats is added to provide implementation based on Monai bundle.

Utilizes DataAnalyzer functions from Auto3DSeg. Changes to DataAnalyzer are supposed to not change the behavior.

TODOs

• support histograms on different features, i.e. image channels.
• update docstrings

Types of changes

• [x] 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.