RBAC: add default role to user

[flaneur2020]

Summary

an user can set up a default role with the statement:

ALTER USER ... WITH DEFAULT_ROLE = '<role>';

also the default role can be viewed in the SHOW USERS statement.

on setting a default role, the role MUST be one of the roles the user has been granted.

on an user connecting to databend-query, she'd get set the default role to the current role in the session (with a check about whether the user still has the role).

on revoking an role from the user, if the role is set as default role, the default role would be reset to None.