[CLI] Add support for NodesThreadsMap in Backup service for Couchbase-cli

Allowing the number of threads per backup service nodes is implemented in the ticket linked.

Currently we are adding a map for threads per node as an API. So customers can use this API if needed. We could allow customers to use couchbase-cli as well to create and alter this nodes threads map but we created a separate ticket to add it as it is not needed for the functionality of the improvement.