Add Feature Groups to Imputers

[mmschlk]

Imputers should be able to handle groups of features instead of single features. A feature group should be treated as one single player. This may be implemented on a shapiq.Game level or on shapiq.Imputer level.

It is also fine if certain imputers disallow this capability (I.e conditional imputer).

I think the marginal imputer, baseline imputer and TabPFNImputer should have no problem getting this to work in.

On that note this would also be nice to allow marginal and baseline imputer to not only work with tabular data (I.e baseline imputer for using mask_tokens for transformers or gray pixels for images).