[New Platform] API needed for referencing the path.data value from config

[tsullivan]

Describe the feature:

In order for plugins to know where to write data to the filesystem as needed, the plugin needs to know the value of the path.data config setting. This value needs to be exposed in the Kibana New Platform somehow.

Describe a specific use case for the feature:
Reporting plugin will use path.data to extract the headless browser executable at start time.

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[joshdover]

This already exists:

class MyPlugin {
  constructor(initContext: PluginInitializerContext) {
    // Creates an observable for just the path.data config
    this.pathData$ = initContext.config.legacy.globalConfig$.pipe(
      map(c => c.path.data)
    );
  }
}