Add a PENDING status for workflows

Current situation

When using the /workflows/{workflow_id}/status(GET) endpoint, a details.status field is returned, containing one of the following: RUNNING, DONE, or FAILED. There is no way to distinguish easily from a workflow that has yet to have one of its job in progress and a workflow that has at least one of its job that is either in progress or already completed.

Desired outcome

A new possible details.status value, PENDING. It denotes a workflow that has been received, but that hasn't started yet.

A workflow is started if at least one of its job is running or has been completed. For example, a workflow including a generator job is considered started if this generated job is running or has completed. A workflow including a 'regular' job is considered started if the job has been assigned an execution environment.

A job that is waiting for an execution environment is not considered started. A conditional job that has been skipped is not considered started either.