Get Envelope metadata from DescribeCoverage for native CRS

[njanakiev]

I think it would be useful to provide the functionality to get the gml:Envelope metadata additionally to the gml:RectifiedGrid. This would return the native CRS of the layer (for version 1.0.0 and 2.0.1) and the accurate axis labels (for version 2.0.1) that can be further used in GetCoverage when using the subset parameter.

Here is an example of the USGS Elevation National Map WCS endpoint with DescribeCoverage:

<gml:Envelope 
  srsName="http://www.opengis.net/def/crs/EPSG/0/4326" 
  axisLabels="Lat Long" 
  uomLabels="Deg Deg" 
  srsDimension="2">
    <gml:lowerCorner>-90.0 -180.0</gml:lowerCorner>
    <gml:upperCorner>90.0 180.0</gml:upperCorner>
</gml:Envelope>