Allow labels for any workflow instructions

Current Situation

• A number of API calls allow to specify labels, for example /workflow/skip.
• A number of API calls require to specify positions, for example /workflow/stop and /orders/add.

Desired Behavior

• Any API calls that are focused on workflow positions should accept to specify a position or a label.
• Labels can optionally be added to any instructions of a workflow from the GUI.
• The Workflow JSON Schema should allow to specify labels for any instructions in a workflow

Maintainer Notes

The following APIs accept also labels instead of positions

• ./orders/add
• ./orders/resume
• ./order/variables
• ./daily_plan/orders/modify
• ./workflow/stop
• ./workflow/unstop