rest: endpoint to fetch a single workflow

[mvidalgarcia]

This is needed to trigger updates in the UI without fetching all the workflows.

We can also take the opportunity to aggregate additional data that now is being fetched in separate requests from the workflow details page:

• /api/logs
• /api/workspace
• /api/specification

To take into account: reanahub/reana-ui#142 (comment)

[mvidalgarcia]

This was partially covered as part of reanahub/reana-ui#229.

Integrating the logs, workspace, and specification in a single endpoint could be considered but for the time being it's better to keep these calls separated to handle the requests asynchronously, as this information is displayed on different tabs on the UI.