Support Getting Cluster Version via SDK, handling authentication for 5.0 RBAC

It would be helpful to have a standard method for getting the cluster version in the SDK.  To date this has been done independently by the Linq2Couchbase client.  However, this has resulted in some code duplication between them.

Additionally, RBAC in 5.0 now requires authentication to access the /pools endpoint to get the version.  Since none of the authentication information from the SDK is publicly accessible, Linq2Couchbase currently has no way to get the cluster version (unless an out of band method for providing authentication to the LINQ client was provided, which would be cumbersome).