Refactor discovery to use invokeInWorkspace 

This is a major set of changes and should be released as a minor version.

 - Refactor `invokeInWorkspace()` to support all `dataSource` methods
 - Implement `getSchema()` and `discoverModelDefinition()` using `invokeInWorkspace()`
 - Add new `dataSourceDef.createModel()` to create model definitions + configs using a discovered model definition from `discoverModelDefinition()`

/to @bajtos @raymondfeng 
/cc @seanbrookes 
> The current version allows you to run bin/datasource-invoke.js manually from command-line, I found it useful for troubleshooting and debugging.

In another PR perhaps... since all output is now logged from the workspace process, I didn't find the need to run the process from the CLI. You could argue this could be useful for the entire WS, but not a priority IMO.