Increase DX of entity events

[MaxSchaefer]

A cleaner way is to

• create a new EntityAggregateBase type, which embeds AggregateBase
• EntityAggregateBase then implements a custom Save() method, which does the same as SaveEntityEventForAggregate
• TaskTemplateAggregate, BedAggregate etc embed EntityAggregateBase instead of AggregateBase
• handler call NewTaskTemplateAggregate(templateID).Save(...)

https://github.com/helpwave/services/pull/884#discussion_r1842406765