Adding optional default attributes for user (namespace, role, warehouse)

When using other tools besides SF console, not always is clear where to define the role and if these parameters are not defined then PUBLIC role is used (which has no permissions and thus probably the user cannot actually do anything).

Introducing:

• Ability to add default_ns for a user to specify its default namespace (database.schema).
• Ability to add default_role for a user to specify its default role.
• Ability to add default_wh for a user to specify its default warehouse.
• Updated README.md with example
• Updated tests

TODO:

• Check for typos on the name of these parameters -> It's not a blocker as Snowflake itself doesn't check for it, but would be nice to avoid issues in the future.