Use JSON column type.

[sinaso]

This is not tested. I have created this only for discussion at the moment.
Does it make sense to add JSON type? The use case is to allow bigquery stream loader to create a column of type JSON (instead of String) when schema is empty.

please check: snowplow-incubator/snowplow-bigquery-loader#385

[snowplowcla]

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[sinaso]

signed.

[snowplowcla]

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