Use JSON column type.

[Sina Sohangir (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/snowplow-bigquery-loader#385

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[Sina Sohangir (sinaso)]

signed.

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