Attribution control support custom

[deyihu]

fix #1320

  const map = new maptalks.Map("map", {
            center: [120.5965, 31.2189],
            zoom: 13,
            bearing: 0,
            pitch: 0,
            zoomControl: true,
            spatialReference: {
                projection: "EPSG:3857",
            },
            attribution: false


        });




        const layer1 = new maptalks.TileLayer("base1", {
            urlTemplate: "https://webrd01.is.autonavi.com/appmaptile?lang=zh_cn&size=1&scale=1&style=8&x={x}&y={y}&z={z}",
            subdomains: ["a", "b", "c", "d"],
            attribution: "&copy; <a href='http://osm.org'>OpenStreetMap</a> contributors, &copy; <a href='https://carto.com/'>CARTO</a>",
        }).addTo(map);

        const control = new maptalks.control.Attribution({ custom: true });
        control.addTo(map);

        control.setContent('hello hello hello');