[openstreetmap/openstreetmap-website] Implement JSON support for the changeset download API (PR #5973)

[mmd]

mmd-osm left a comment (openstreetmap/openstreetmap-website#5973)

> > Changefiles today are usually sorted by (OSM type, OSM id, version). This ensures that most(*) dependent changes are already available, when reading an object. Consumers may rely on that order.

>  I think we can keep the sorting of objects (per action) as is, regardless of the output format (ie. for the changeset download case).

Ok, I guess my comment was a bit premature. This happens when commenting on a mobile without checking the actual implementation...

Sorting today is based on timestamp, object version, type (node/way/rel), and object id ([souce](https://github.com/zerebubuth/openstreetmap-cgimap/blob/v2.0.1/src/osmchange_responder.cpp#L32-L62)). The action itself doesn't matter. It will be determined for each single element ([source](https://github.com/zerebubuth/openstreetmap-cgimap/blob/v2.0.1/src/osmchange_responder.cpp#L188-L207)).

As can be seen in the following examples, the current sorting wouldn't work anymore and re-grouping by action would be mandatory for the proposed JSON format in this PR. API clients would definitely see a fairly different sorting in this case.

* https://www.openstreetmap.org/api/0.6/changeset/52067542/download
* https://www.openstreetmap.org/api/0.6/changeset/52067546/download
* https://www.openstreetmap.org/api/0.6/changeset/52067549/download

I think, my original proposal in https://github.com/zerebubuth/openstreetmap-cgimap/pull/407 would avoid this issue. Iit  uses the exact same sorting as today without a need to regroup the data.


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