Add links with integrity to custom projections instead of inline modules which are not recommended

[prushforth]

Currently, when you set the default projection of a layer to a custom MapML: projection, the preview document contains an inline script like so:

<script type="module">
let customProjectionDefinition = `
{
"projection": "USNATATLAS",
          "origin": [-8046094.80800152, 13563887.19875385],
          "resolutions": [62900.55278081377, 31450.276390406885, 15725.138195203443, 7862.569097601721, 3931.2845488008606, 1965.6422744004303, 982.8211372002152, 491.4105686001076, 245.7052843000538, 122.8526421500269, 61.42632107501345, 30.713160537506724, 15.356580268753362, 7.678290134376681, 3.8391450671883405, 1.9195725335941702, 0.9597862667970851],
          "bounds": [[-8046094.80800152, -2538654.31313448], [8056446.70388680, 13563887.19875385]],
          "tilesize": 256,
          "proj4string" : "+proj=laea +lat_0=45 +lon_0=-100 +x_0=0 +y_0=0 +ellps=sphere +no_defs +type=crs"
}`;
let map = document.querySelector("mapml-viewer");
let cProjection = map.defineCustomProjection(customProjectionDefinition);
map.projection = cProjection;
</script>

This generates a warning in devtools, like this:

It should be possible for geoserver to generate that as follows:

<script type="module" integrity="sha384-1/70nM+8Kdg0K81g8Vb5I+/UaWfKy/gd5CiSgsEZsabsdU68XX7tfQi5iMMZGdOp" src="http://example.org/geoserver/web/wicket/bookmarkable/org.geoserver.web.demo.SRSDescriptionPage?10&code=EPSG:2003"></script>