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
• [x] boolean
• [ ] int8
• [ ] int16
• [ ] int32
• [x] int64
• [x] double
• [x] decimal4
• [x] decimal8
• [x] decimal16
• [ ] date
• [ ] timestamp
• [ ] float
• [ ] binary
• [x] string