`Sortinganalyzer.select_channels()`: update select_channel_extension_data for waveforms/PCA

[alejoe91]

• make sure sparsity is sliced according to channel_ids, see Add SortingAnalyzer.select_channels() function #4648 (comment)
• make it private, since select_channels is only supposed to be used with the split_by functionality when the SortingAnalyzer comes from aggregated sortings/recordings