Add option to authenticate with redis servers

[stewrutledgebonnier]

We have all of our redis instances setup with authentication which makes using them as a backend impossible.

This change just adds AUTH support to the redis client, based on the presence of password in the configuration file.

[CLAassistant]

All committers have signed the CLA.

[jranson]

@juliusv Any concerns about this one (e.g., storing passwords in plaintext)?

[SuperQ]

Storing credentials in a file is the preferred thing these days. LGTM.

[stewrutledgebonnier]

I didn’t really see any existing convention for secrets in this project and it seemed like adding support for config via env variables or similar solutions is out of scope for the actual issue (using password protected redis)

[SuperQ]

ENV vars are considered unsafe compared to files.

[stewrutledgebonnier]

Not implying otherwise, I just mean that I used the existing configuration convention instead of attempting to implement any new ones.