Avoid boxing/unboxing in VBufferDataFrameColumn #6864

VBufferDataFrameColumn stores public readonly struct Vbuffer items.
This column is implemented based on current StringDataFrame column implementation, however any string is a reference type and any Vbuffer is a value type, so there are several issues with current implementation. One is boxing/unboxing that happens on ever access by row index, another - unability to store null elements (#6852)

The goal of this issue is to avoid boxing/unboxing on indexing and throw NotSupported exception with clear explanation instead of null reference exception