Add support for Snowflake SSO authentication round 2

[adrianisk]

@cmcarthur Re-do of PR on the dev/stephen-girard branch #1185

Feature request #1172 - adds support for SSO authentication through the Snowflake Python connector, more details in the issue. I updated the Snowflake connection contract to add an 'authenticator' field, then just updated the Snowflake adapter to pass in whatever combo of authentication args was specified to the connector. Validation of the actual args is left up to the connector itself, which gives good error messages when they're invalid.

Unit tests verify everything's getting passed through correctly, and I manually tested that I can authenticate through both the 'externalbrowser' and Okta authenticator.

I did remove 'password' from the required fields in the Snowflake contract, but the error message returned by Snowflake when you forget it is pretty good:

ERROR: Database Error
251006: None: Password is empty

[cmcarthur]

@adriank-convoy thanks for the contribution! we'll get this into the next release (Stephen Girard)