feat: Allow customization for BelongsToSelect label

[pxlrbt]

As discussed on Discord this allows to easily set a label for the BelongsToSelect without overwriting the whole getSearchResultsUsing() and getOptionLabelUsing() callbacks.

Outlook
This could also applied to BelongsToManyMultiSelect and BelongsToManyCheckboxList . Both are missing the searchable() method or $searchColumns property this feature relies on. While it makes sense to add this to BelongsToManyMultiSelect there is no search on BelongsToManyCheckboxList and we need to define the fields for the SELECT statement differently.

[danharrin]

Thanks!

[pxlrbt]

Thanks for your additions and merging!