feat: Groups (Marker-Mode): Backend

[resah]

This is part of the feature "Groups":
To visualize teams, or as we call it at AOE "circles", it should be possible to define groups. To accommodate for different use cases, there are two different variants for grouping. This feature is part of "Marker groups (map)":
Markers inherently belonging to a group by their attributes, like

• all markers with attribute team and value Space invaders
• all markers with attribute post and value Developer
• all markers with attribute contents and value Club-Mate

---

Backend

• Endpoints
  • GET /api/markers/attributes - list all marker attributes (ignoring marker type)
  • GET /api/markers/attributes/:attribute/values - list all existing values of the given marker attribute (ignoring marker type)
  • GET /api/markers/filter?attribute={attribute}&value={value} - list markers, which have given attribute and attribute value