Add AVRO as a valid BigQuery SourceFormat

[objectfox]

Fixes#2627, and brings the function up to date with the BigQuery Documentation.

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[objectfox]

I signed it!

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CLAs look good, thanks!

[tseaver]

@jeffk Thanks for the patch! Interesting that we had support for it for the desitnationFormat field for extracts, but not the sourceFormat field for loads: I almost feel like we've been gaslighted again.