Add private key path

[ahuang11]

User wants to use Snowflake dbt profile, but private_key doesn't work because it's private bytes.

Instead the profile expects private_key_path and private_key_passphrase: https://docs.getdbt.com/reference/warehouse-setups/snowflake-setup#key-pair-authentication

Originally from https://prefect-community.slack.com/archives/CL09KU1K7/p1671766851641279

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