[Pt2(.mu.-S)2(PPh3)4] as a Metalloligand toward Main-Group Lewis Acids. The First Square-Pyramidal Structure of an Indium(III) Complex Adduct in (PPh3)4Pt2(.mu.3-S)2InCl3 and Tetrahedral Gallium(III) in the Ion Pair [(PPh3)4Pt2(.mu.3-S)2GaCl2][GaCl4]