Do not flatten nested objects when sending data to Snowflake

Currently `target-snowflake` flattens all nested objects if such are present in catalog of a tap. This is different behaviour from how Stitch platform handles sending data to Snowflake - I've seen the latter sending VARIANT-type data to Snowflake, without flattening them. 

Might be worth synchronising this behaviour with Stitch, or maybe allowing for configuring it.