Adding username as parameter for the redis authentication

[kerkmann]

Intention

I am using Authentik which is using your library. There is an open ticket for settings the Redis username. Sadly the username is not set in this library. Without fixing that issuer here, it's impossible to fix it in Authentik. ^^"

Changes

Adding the username as optional parameter.

I hope the PR is looking good, I am not a go developer. Please let me know if something is wrong/missing.