Support for Training processes

Follow-up of #1288 (closed)

• Add training mode to the frontend enum - frontend!1308 (merged)
• Add training mode to tasks enum - tasks!316 (merged)
• Serialize training attributes in endpoints returning processes

assigned to @vrigal

mentioned in merge request !1773 (merged)

Thanks for that, especially theh serializarion part

mentioned in merge request !1780 (merged)

changed the description

mentioned in commit cb8d16be

closed

mentioned in merge request !1797 (merged)