`get_feature_names_out` for `EstimatorTransformer`

[CarloLepelaars]

This PR adds get_feature_names_out functionality for EstimatorTransformer. This is a desirable method for EstimatorTransformer because its output is used as input for a subsequent estimator.

• [x] Implement get_feature_names_out for EstimatorTransformer.
• [x] Write core tests for implementation.
• [x] Create tests to ensure get_feature_names_out works with a FeatureUnion component.
• [x] Research if get_feature_names_out implementation is necessary and desired for other Meta estimators.

@koaning I think it is not necessary to implement get_feature_names_out for the other estimators in sklego.meta, because it seems all these estimators are used as the last component in a Pipeline, except for EstimatorTransformer. EstimatorTransformer will often be a component somewhere in the middle of a Pipeline and therefore get_feature_names_out is desirable. That said I'm happy to work on other implementations of get_feature_names_out in sklego.meta if you disagree with this reasoning.

Solves Issue #533

[CarloLepelaars]

Hey, yes this feature is ready and meant to be reviewed. I'll make sure to add [WIP] if something would be still work in progress

As for the commits there are some commits here that are already merged to scikit-lego from a previous PR, but somehow it still shows when I create a new local branch. Everything up to commit #f06a7af is duplicate. Do you know how we can clean up the commit history?

Otherwise I think we are ready to merge! Fixed the minor nitpicks.

[CarloLepelaars]

@koaning, shall we go ahead and merge this one to main?