Rename imputer classes and add deprecation aliases

[aniq63]

What this PR does

This PR renames the imputer classes requested while maintaining backward compatibility through a deprecation cycle.

Fixes #971

Changes

• Renamed MeanMedianImputer to MeanImputer
• Renamed AddMissingIndicator to MissingIndicator
• Renamed ArbitraryNumberImputer to ArbitraryImputer
• Added deprecated aliases with FutureWarning
• Updated package exports
• Updated tests to cover both the new and deprecated class names, including deprecation warnings

Testing

• All updated tests pass successfully.

[solegalli]

Hi @aniq63

Thanks a lot for your contribution.

We need to split this PR into 3, one per class.

We also need to update the documentation relevant for each transformer, which you'll find inside the docs folder. Basically, we need to replace all calls to the old transformer names by the new transformer names.

We also need to rename files from if necessary, for example if a file was called meanmedianimputer.py or .rst, we need to rename that to the new name of the transformer.

There is also this PR that adds the same changes #972, so maybe you could start with MissingIndicator, and we take it from there?

Note that the classes are tested in additional files and need to be updated there too: (tests/check_estimators_with_parametrize_tests.py, tests/test_imputation/test_check_estimator_imputers.py)