Bugfix: Populate missing name/id attributes in apstra_datacenter_system data source

[chrismarget-j]

The apstra_datacenter_system takes either name or id as input and populates system details in an object called attributes.

The attributes object has both name and id inside.

Prior to this PR, nothing was populating the top-level (input) name or id other than the user. One of those two fields (whichever the user didn't use) would always be Null when the data source returned.

This PR ensures that the top-level name and id fields are both always populated making it safe for the user to refer to data.apstra_datacenter_system.<name>.id, rather than data.apstra_datacenter_system.<name>.attributes.id

Also, the ReadFromApi() method has been renamed AttributesFromApi() to make it clear that only the attributes object is being populated.