Add ability to restore a specific keyspace

[c3-clement]

What is missing?

As of today, MedusaRestoreJob doesn't allow to restore a specific keyspace.
It would be great to be able to specify a list of keyspaces that should be restored.

Why do we need it?

In my company, we want to implement multi-tenancy environments. In the Cassandra layer, each tenant would be represented by a keyspace.
Therefore, we need to be able to restore a single keyspace, without downtime on other tenants.

Anything else we need to know?:

I've seen this issue k8ssandra/k8ssandra#222 , but I couldn't find the ticket in K8ssandra project board.

It seems that the medusa cli allows to specify keyspaces to restore through the --keyspace argument
https://github.com/thelastpickle/cassandra-medusa/blob/master/docs/Restoring-a-single-node.md

┆Issue is synchronized with this Jira Story by Unito
┆Issue Number: K8OP-287