Provide JSON API for search

(Reopening your issue here because we moved development @stefan-niedermann :)

There should be an API for searching for addresses.
Otherwise every client would have to implement searching in contacts etc. itself. Providing an API would save every client much work.

This should be read as "WiP", just brainstorming.

Request

GET /search?q=<string>

Response

Nextcloud Maps should search all reachable information in the Nextcloud (especially from Nextcloud Contacts, but maybe from Nextcloud Calendar - Meetings in the near future?). And it should search osm via nominatim to get PoIs.
Mimetype: application/json

{
  /* Contacts from contacts app */
  contacts : [
	  {
		firstname : <string>,
		lastname : <string>,
		address : {
		  nr : <int>,
		  street : <string>,
		  plz : <int>,
		  city : <string>
		},
		lat : <int>,
		long : <int>
	  },
	  ...
  ],
  /* Points of Interest (bars, clubs, toilets, buildings, ...) */
  poi : [
      {
        title : <string>,
        type : <string>, /* Enum? */
		address : {
		  nr : <int>,
		  street : <string>,
		  plz : <int>,
		  city : <string>
		},
        lat : <int>,
        long : <int>
      },
      ...
  ],
  /* Addresses from OSM search */
  addresses : [
    {
      /* Whatever the OSM JSON-API delivers ;-) */
    },
    ...
  ]
}

No repository files, tests, or entry points are named. Start by locating the existing API and search integrations, then review how Nextcloud Contacts and OSM Nominatim could be queried. Done would require an agreed scope and response contract for contacts, points of interest, and addresses, followed by an implemented and tested GET /search endpoint.