Add data source to fetch list of all data centers

[jimeh]

We have the singular katapult_data_center data source already, which fetches details about a specified data center. To complement this, we should also add a plural form which returns a list of all data centers.

This should help improve visibility of data centers, avoiding issues like #81.

Tasks

• Add katapult_data_centers data source, which returns a list of data centers.