Add a helper function for connecting to Snowflake.

[atheriel]

This commit introduces a new odbc::snowflake() function intended to help setting up connections to Snowflake and handling auth correctly on platforms that provide Snowflake-native OAuth credentials. It generally follows the pattern established with odbc::databricks().

This is pretty experimental, I'm not sure what the Workbench interface is going to look like and Snowflake's container platform is still in private preview, so how it manages tokens might be subject to change. I'm putting it up here so interested folks can see what things could look like.