Add username with password authentication

[michilehr]

Since Redis 6, the auth command now also accepts a user

With this change, a username can be passed with -u or --username=

Example:

perl check_redis.pl -H redis --password="my-password" --username="my-username"

A username can also be passed via the credentials file in the second line so there are no breaking changes.

my-password
my-username