Add 'default-input' flag to %%bq execute

[rajivpb]

This is to enable a progressive pipeline-authoring experience with the ability to test individual cells before running them in the pipeline. This way, a user can define SQL in the form "SELECT .... FROM input WHERE ...", and also execute the SQL in the %%bq execute cell, thus increasing confidence in the success of the %%bq pipeline cell.

From feedback with @lakshmanok

[nikhilk]

I think we discussed updating %bq execute that currently takes in data sources to have some mechanism for taking in an 'input' data source... so +1 to this.