Connect map layers (data) with issues through custom fields

[dkastl]

Problem

The recent release supports a variety of map layers, some of them like KML or GeoJSON layers for example can provide data from external sources like facility or vehicle locations, POI's, etc..

Beside showing them as an additional layer it would be nice to relate issues to specific layer items. Some examples:

• A layer of service vehicles with live location updates could relate a specific vehicle to an issue.
• A layer of facilities (like hospitals, clinics) could relate a specific facility to an issue

That way an item of the map layer becomes a geo-referenced attribute of an issue. Additionally these data can be fetched from external resources.

This could also solve gtt-project/redmine_supply#13 when supply items or assets are managed externally.

Improvement

The case above could be implement by connecting a suitable map layer (i.e. vector layer type) to a custom field of type "list".

1. Create a new custom field of type list (simple list or key value list)
2. Provide configuration for suitable map layers to
   • connect the layer to a custom field
   • specify layer attributes to be used for list (keys and) values (JSON Path?)
   • optionally fetch list items from map layer and configure the custom field accordingly
   • add option how to show the layer on the issue page (i.e. highlight selected item, only show the selected item).
   • optionally use the Link values to URL field in the custom field to provide a URI to a map layer item.
3. Add map layer <-> custom field interaction on issue(s) page

Unknowns

• How can we ensure lists to be in sync with map layers?
• Can we support lists with multiple selections?