Introduce WFS search

[dnlkoch]

This includes a generic WFS search option to the MultiSearch component while refactoring it to become no longer strongly linked to specific search engines.

To activate:

{
  "name": "search",
  "config": {
    "engines": [
      "nominatim",
      "wfs"
    ]
  }
}

The existing configuration for the layers in untouched and supports solr and wfs the same way:

{
  "searchable": true,
  "searchConfig": {
    "attributes": [
      "NAME",
      "scalerank"
    ],
    "displayTemplate": "Result: {NAME}"
  }
}

[github-actions]

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