Add additional cloud provider driver "helper" functions

[atheriel]

The odbc::databricks() and odbc::snowflake() functions have been pretty successful, because we've been able to stuff a lot of unpleasant auth and configuration details in there that users would previously have to copy around manually.

There are several other obvious candidates for "cloud provider helper functions", too:

• redshift() and rds() helpers that handle AWS IAM credentials and maybe integrate with the paws package; and
• A sqlserver() helper that picks up on Azure credentials.

I think we should try to build these out over time.