Add authentication and user management support for TypeDB Cluster (#157)

## What is the goal of this PR?

We have added authentication and user management support for to TypeDB Cluster.

## What are the changes implemented in this PR?

- Added `--username` and `--password` options. The options are validated such that they can only be supplied when connecting to TypeDB Cluster (it doesn't make sense to supply them when connecting to TypeDB which doesn't support authentication).
- Added the user management feature allowing users to manage user credentials:
  - `user create user1`
  - `user list`
  - `user delete user1`

Author: Ganeshwara Hananda