Add support for Memorystore Redis AuthString

[szydek]

What problem are you facing?

GCP Memorystore Redis supports the "Auth String" configuration:

https://github.com/googleapis/google-api-go-client/blob/39fb9f0d81a68916f0849eb9410d8a4b7291b011/redis/v1beta1/redis-gen.go#L655-L679

How could Crossplane help solve your problem?

Crossplane users could benefit by having the auth string configured as part of deployment and stored as a k8s secret to help secure GCP Redis instances. If deploying via Crossplane, it would be ideal not to have to secure the instance outside the Crossplane workflow.

[turkenh]

@szydek could you please explain what is the difference with this issue and what you've added with #334 ?

I am understanding that this issue is about taking AuthString as an input via a k8s secret whereas #334 is about the other direction, e.g. fetching gcp generated auth string into connection secret. Is it correct?