Primitive type support in open-variant

[wjones127]

Full list here: https://github.com/apache/spark/tree/master/common/variant#encoding-types

Implement methods to extract from VariantRef and write each of these into buffer (write_*) as well as methods on ObjectBuilder and ArrayBuilder:

• null
• boolean
• int8
• int16
• int32
• int64
• double
• decimal4
• decimal8
• decimal16
• date
• timestamp
• float
• binary
• string