PLT-1016 - Add methods for External Workforce management

[paulnoirel]

This PR adds three methods to manage external workforces (aka BPO organizations) from a project, in particular:

project.add_external_workforce(workforce_id)

project.remove_external_workforce(workforce_id)

project.get_external_workforces()

All three methods return a list of ExternalWorkforce associated with the project at the end of the operation or an empty list.

[adrian-chang]

See comments

[lnash4]

so per @manuaero 's request we would not like to use the add and remove endpoints anymore, now that we have built boost requests within the product

the mutation's name is upsertProjectBoostWorkforce, this will automatically add the alignerr org as an external workforce

then if the project is properly set up, which means they have

1. ontology configured
2. data added
3. there are labeling instructions for the ontology

then you can move the project to the 'REQUESTED' status with the
updateProjectBoostWorkforceStatus mutation

[paulnoirel]

I am closing the current PR since it is not relevant with Alignerr.