BigQuery: Node.js - Implement Scripting API Changes

[d-torres]

This is a tracking FR for implementing Scripting API Changes within the BigQuery API.

This work complements the Routines API, which allows for the creation/persistence of stored procedures.

Scripts are a form of query that can contain multiple SQL statements, and features such as logic, variable assignments, and related control flow functionality. General documentation about scripting can be found in the Standard SQL reference pages.

Reference implementations: Go, Python and Java.

[steffnay]

Fixed with #580