Delay start of engine

[ahuang11]

Delay start of engine until needed like PrefectHQ/prefect-snowflake#58

Useful in cases where user simply wants to save block or use in prefect-dbt

Example

Screenshots

Checklist

• References any related issue by including "Closes #" or "Closes ".
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• Includes tests or only affects documentation.
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[ahuang11]

These will be set to None:

        data.update({k: None for k in {"_engine", "_exit_stack", "_unique_results"}})

And when get_engine / get_connection is called, it'll be back to normal.