Implement CacheSubnetGroup resource for ReplicationGroup to be created in a VPC

[muvaf]

What problem are you facing?

Replication Group resource can be created in a VPC only if you create a cache subnet group and give its name during creation as opposed to EKS where you create individual subnets and give their IDs.

How could Crossplane help solve your problem?

It seems like CacheSubnetGroup is a logical resource where you can either create new subnets during creation or choose from existing ones. So, it's a bit tricky in Crossplane where every managed resource is independently managed. Implementation of that resource would probably look like Azure's resource groups where its sole interface is to list/add/remove the downstream resource.

[muvaf]

Fixed by #186