Add an asynchronous implementation of `GlobalCache` to support Valkey

[Niorlys]

Is your feature request related to a problem? Please describe.
Since Google Cloud has Memorystore for Valkey, we are thinking of using Valkey for caching ndb entities, but specifying a Valkey client as RedisCache often results in valkey.exceptions.WatchError: Watched variable changed., which bubbles up into our app when fetching entities, so it could be a good idea to add an implementation for Valkey.