Ask for the operator of a post box if the tag is missing on the node

Every time I create a new post box I end up manually going into the tags of the node and adding the operator by hand. Depending on the area this is important info since multiple postal companies can operate in the same area (like mine) and they obviously won't accept each other's stamps. ### General Affected tag(s) to be modified/added: https://wiki.openstreetmap.org/wiki/Tag:amenity%3Dpost_box https://wiki.openstreetmap.org/wiki/Key:operator ### Checklist Checklist for quest suggestions (see [guidelines](https://github.com/streetcomplete/StreetComplete/blob/master/QUEST_GUIDELINES.md)): - [x] 🚧 To be added tag is established and has a useful purpose - [x] 🤔 Any answer the user can give must have an equivalent tagging (Quest should not reappear to other users when solved by one) - [x] 🐿️ Easily answerable by any pedestrian from the outside but a survey is necessary - [x] 💤 Not an overwhelming percentage of quests have the same answer (No spam) - [x] 🕓 Applies to a reasonable number of map data (Worth the effort) ### Ideas for implementation Any node that is tagged amenity=post_box but lacks the operator key triggers this quest. The answer is provided as written text, but giving suggestions based on a list of big postal service companies and previously submitted answers by the user can help out. It should have a higher priority than the current collection_times quest.