separate crs registration from resource extent

currently the crs of the resource extent is used to indicate the crs of the resource

pygeometa/pygeometa/schemas/iso19139/main.j2

Line 118 in ea4c94d

    
           <gco:CharacterString>urn:ogc:def:crs:EPSG:{{ record['identification']['extents']['spatial'][0]['crs'] }}</gco:CharacterString>

many times this is not the case, the resource extent is generally expressed in wgs84, where the crs of the resource is typically different

Can we add a key to spatial indicating the crs of the resource

this could be ok, if we state in docs that users always need to place the bounds in the crs of the source (which makes sense)

but then i would expect a coordinate transformation to wgs84 in the extent, so the spatial extent is advertised in wgs84, currently it renders a polygon in the source projection, which is hard to read for most software clients

<gmd:polygon>
                <gml:Polygon gml:id="P001" srsName="3035" srsDimension="2">
                  <gml:exterior>
                    <gml:LinearRing>
                      <gml:posList srsName="3035" srsDimension="2">1500000 900000 1500000 5500000 7400000 5500000 7400000 900000 1500000 900000</gml:posList>
                    </gml:LinearRing>
                  </gml:exterior>
                </gml:Polygon>
              </gmd:polygon>