Add redshift_namespace data source

[sworisbreathing]

As discussed in #12, here is the PR to add a redshift_namespace data source. The new data source has no additional attributes - the namespace will be set in the ID. It is intended for future use cases such as data sharing (which I plan to start working on next)

Example:

data "redshift_namespace" "producer" {  ​
}

output "producer_namespace" {
  value = data.redshift_namespace.producer.id
}