[ENH] __getitem__ aka [ ] dunder for transformers, column subsetting

[fkiraly]

This PR adds column subsetting functionality to transformers via the [ ] (__getitem__) dunder.

Similar to how this works in pywatts, using the [ ] dunder will change any transformer to additionally subsetting columns of outputs (first key) and/or inputs (second key).
This is done by pipelining with the existing ColumnSelect transformer, using the existing TransformerPipeline.

Examples:
my_trafo["a"] is shorthand for ColumnSelect("a") * my_trafo, which in turn is shorthand for TransformerPipeline([ColumnSelect("a"), my_trafo])
my_trafo[idx1, idx2] is shorthand for ColumnSelect(idx1) * my_trafo * ColumnSelect(idx2), which in turn is shorthand for TransformerPipeline([ColumnSelect(idx1), my_trafo, ColumnSelect(idx2)])

Includes tests.

Relies on:

• [ENH] extend ColumnSubset to work for scalar columns parameter #2906, so subsetting like my_trafo["a"] is valid (same as my_trafo[["a"]])
• [ENH] transformer vectorization: ensure unique column names if unvectorized output is multivariate #2958, for an example where input subsetting and output subsetting do not commute
• [ENH] fix pipeline vectorization for univariate estimators #2959, since vectorization for the ThetaLinesTransformer in a pipeline was not working

Credit to pywatts for the idea - thanks, @benHeid, @SMEISEN, @kalebphipps!

Also see discussion here: #2654

[fkiraly]

Credit to pywatts for the idea - thanks, @benHeid, @SMEISEN, @kalebphipps!

Also see discussion here: #2654