Add auto generated code for "system" datasource

Code was generated using containerized version of
tfplugingen-openapi (v0.3.0) and tfplugingen-framework (v0.4.0)

The general form of the commands was:

```
$ tfplugingen-openapi generate -config config.yaml spec.yaml
$ tfplugingen-framework generate all \
    --input provider_code_spec.json --output . --package server
```

The relevant scripts and OpenAPI specs are stored in private git repositories
for the time being.