Add namespace parameter to AstraDB integration

[renaisser]

Describe the solution you'd like
When you use a database for different clients, the correct approach is to create a different namespace for each client. This is extremely useful for scaling and keeping the clients isolated.

Additional context
I have noticed in the code that you have an AstraClient class where the namespace already exists. I think it would be relatively quick to include namespace parameters in the AstraDocumentStore class.