Missing Structures to publish in APIv2

[Chatewgne]

Describe the bug

Structures appear in APIv2 if they are related to a published Trek or a published Touristic Content. This is not exhaustive, we should also check for 7 other models that are related to Structure : Infrastructure, Signage, POI, Service, Touristic Event, Site, Course.

(As a result, some objects might not appear in GeotrekAggregator because their mandatory structure wasn't found and they aren't downloaded - we have to bypass this by not aggregating the Structure)

Here is the draft for a fix : #4435 but this isn't viable because the request is way too slow.

We need to find a more efficient way to filter for published related objects in APIv2, especially if we have so many different models to check for.

[camillemonchicourt]

This is for /structure/ route ?
Maybe this route could just return all structures in Geotrek-admin instance ?
Or we could add a "published" field on this table ?