Add a data source for listing available versions for Container Engine
clusters or retrieving the latest available version.
This is mostly to support our tests for specifying a version for cluster
creation; the withVersion test has been updated to use the data source,
meaning it will stop failing on us as new versions get released.
* first version of this datasource
* add network and subnetwork datasource and documentation
* modify sidebar reference in documentation
* fix elements after review on network and subnetwork datasources
* fix fmt on Google provider.go
* modify code with the review
* modify documentation layout order
* fix alphabetic order in provider.go
* fix rebase issue and documentation datasource => data