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 
It would be nice to change the code throwing the error so that we don't have to detect it in loopback-workspace at all. I suppose that's work for another pull request.