How to convert city level geojson to country level geojson

Question

I have a GeoJson file that represent a country however that file doesn't contain the polygon representing the country itself rather a list of cities and their respective geometries.

Is there a way to merge all those city geometries into a single polygon representing the country.

Example: Germany

  "type": "FeatureCollection",
  "features": [
    {
        "type": "Feature",
        "geometry": {
            "type": "MultiPolygon",
            "coordinates": [...]
        },
        "properties": {
            "name": "Berlin",
            "description": "test"
        }
    },
    {
        "type": "Feature",
        "geometry": {
            "type": "MultiPolygon",
            "coordinates": [...]
        },
        "properties": {
            "name": "Hamburg",
            "description": "test"
        }
    },
    ...

    ]

Is there a method to extract Germany polygon from its cities?

Thanks