Add support for get_select_value operations

I needed to pull some data out of NetSuite, and needed an operation that was not yet implemented. I wasn't sure what object to attach the operation to, so I created `BaseRefList` - which sounds very generic and difficult-to-discover, but it somewhat describes the expected results.

```ruby
NetSuite::Records::BaseRefList.get_select_value(
  field: 'class',
  recordType: 'salesOrder',
  sublist: 'itemList'
)
```

Right now there's no tests (I assure you, the example works), so this still needs some attention. I wanted to open the Pull Request early to solicit feedback.