Hydrothermal Synthesis and Crystal Structure of One Rare Earth Substituted Keggin-Type Germanotungstate [Cu(en)2]2[Cu(en)2(H2O)]2H3{[Cu(en)2]2[Na2(H2O)1.75][K(H2O)3][Dy2(H2O)2 (GeW11O39)3]} · 6H2O