Add support for a credential provider

[trentbitterman]

Currently, coredis allows authenticating with Redis clusters by supplying a username and password. This works in most situations, but in some cases, such as an Elasticache cluster using IAM for auth, this can't be used as the "password" will rotate and there's no good way in coredis to update the password when it does. Other Redis libraries, such as redis-py, allow the specification of a credential provider, which is basically just an object with a get_credential method that will return the current password when called. It would be useful if coredis had something similar.

Expected Behaviour

In addition to supporting a password string parameter, the various coredis clients should also allow passing in a credential provider object that coredis can use to query the password/credential whenever it is needed.

Thanks for all of your work on this library. It's really great!

[alisaifee]

Apologies for the late response. This makes a lot of sense and would be a pretty straightforward addition to the API. I'm open to a PR for this as I've had limited bandwidth recently for maintenance, however, if not - I'd appreciate any suggestions regarding which providers make sense as builtins versus just being documented as recipes or suggestions in the documentation.

[trentbitterman]

No worries. I'll work on putting something together and hopefully have a PR up sometime next week.