[FEA] StructColumn `__setitem__` and `__getitem__`

[shaneding]

Issue for tracking the implementation of __setitem__ and __getitem__ on StructColumns.

• cudf.Scalar({'a': 1, 'b': 2})
• Add get_element_functor support for struct_view
• Add scatter for struct_scalar
• Implement __setitem__ (python side)
• Implement __getitem__ (python side)
  cc @brandon-b-miller @isVoid