Provide a federated street level imagery substitute for OpenStreetMap contributors

Similar to KartaView, Mapillary, Bing StreetSide, Google StreetView, etc. See:

• https://wiki.openstreetmap.org/wiki/Street-level_imagery_services

In our former use case, we have asked as a favor a person ("remote controlled drone") to scan all streets of a remote town while taking geolocated photographs either automatically or manually. Then the helper uploaded the set of photos to folders. At the end, we've organized "remote mapping parties" where everyone would sync the whole folder to their machines so they could load them into the JOSM editor as an overlay.

This is already quite wasteful, as I don't really want to store the vast amount of images - I usually go through them once and delete them. It would be more desirable if we implemented a new JOSM plugin where we could just add the NextCloud or folder URL and it could stream the photos real time via caching.

Another inefficiency is that in case we split the work by areas based on attendance, it would be sufficient if participants only needed to transfer the photos that they are working on, not the whole town.

For this, little additional development would be required on part of NextCloud. However, if we needed to implement a JOSM plugin like this anyway, it could mind as well be a full featured alternative to Mapillary & KartaView. I.e., based on a globally-federated NextCloud instance (or just a CSV with links to NextCloud instances managed on GitHub, etc), we could visualize the photos of everyone on the map. To make this more efficient, it would be desirable to implement a bit more geo-indexing/sharding/quadtrees/whatnot.

The basic idea would be that the client would have a cached index about which instances of NextCloud have photos within the given zoomed-in area (e.g., graticule), and then the client could ask each instance about the photos they have within the specific boundary box.

The issue names no repository files, tests, or entry points. Start by reviewing the existing Maps architecture and how JOSM, NextCloud, and imagery services could fit; the broad federated imagery proposal needs a concrete scope and acceptance criteria before completion can be defined.