Support preview of table in SQLAlchemy

[riordan]

Bigquery's a big believer in SELECT * as a bad pattern. This totally makes sense. However, folks use SELECT * all the time as a way of getting the schema (don't ask me why).

While fortunately the BQ API affords the tabledata.list method, which can be used to inexpensively preview results, there doesn't appear to be a way to invoke this from within sqlalchemy.

This is particularly pronounced in tools that use sqlalchemy which have the SELECT * pattern baked in as a way to preview tables (I'm looking at you, apache/superset#17299). If there were a way to invoke this via sqlalchemy, this could allow tools like Superset to more effectively use BQ resources (rather than SELECT * from table every time someone starts to work with it in its sql editor).