Delay connection to Snowflake until needed

[ahuang11]

User wanted to be able to use SnowflakeConnector purely to populate the dbt profile and then run dbt; however, in recent PRs, a connection started whenever SnowflakeConnector was instantiated.

This PR delays starting a connection until required.

Originally from https://prefect-community.slack.com/archives/CL09KU1K7/p1672777811532289?thread_ts=1671766851.641279&cid=CL09KU1K7

Example

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• References any related issue by including "Closes #" or "Closes ".
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