Add support for "external" tables

[tseaver]

See the Table resource field and the federated data sources narrative docs.

External tables can be one of:

• Files in Cloud Storage
• Google Sheets
• Bigtable tables
• Datastore backup sets

@jgeewax I'm pretty sure those fields were not in the resource when I first implement v2 last August. Are the back-end team pushing out new features without bumping the API version?

[lukesneeringer]

@tswast I need guidance on this one. Can this be either moved to an overall BigQuery feature doc (either in the pre-beta or post-beta section) or closed if no longer needed?

[tswast]

Should be moved to the doc probably. Definitely still relevant.

…

On Fri, Aug 11, 2017 at 2:15 PM Luke Sneeringer ***@***.***> wrote: @tswast <https://github.com/tswast> I need guidance on this one. Can this be either moved to an overall BigQuery feature doc (either in the pre-beta or post-beta section) or closed if no longer needed? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#2372 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAPHAxl_KeXv_3A3GuSeuLVWo9as7u6Jks5sXMSGgaJpZM4KC6l5> .

[gealvare]

+1 Definitely still relevant.

[LFischerstrom]

I manage to query Avro by doing this:

class QueryJob(bigquery.query.QueryJob):    
    def _build_resource(self):
        return self._properties

[tswast]

Fixed by #4193, to be merged to master in #4245.