Add capability to create module users

[ThatsMrTalbot]

This PR adds a new RedisUser type that wraps the native "RedisModuleUser" type as well as functions to set the ACL on that user.

It also adds methods to the context to authenticate a client using the created RedisUser. This is the foundation for building Redis modules that can do user management (I also would like to implement a solution that wraps RedisModule_RegisterAuthCallback when that is released).

I updated the acl example and tests with the new functionality.

Note: This is my first real play with Rust so any feedback is appreciated.