Support GDAL DEM file URI's

[Ryanf55]

Desired behavior

Allow SDFormat to load a DEM that uses the GDAL virtual filesystem format.

Alternatives considered

• Skip SDF, write a custom plugin

Implementation suggestion

<heightmap>
   <use_terrain_paging>false</use_terrain_paging>
    <uri>gdal:://vsizip/vsicurl/https://terrain.ardupilot.org/SRTM1/ap_srtm1.zip/ap_srtm1.vrt</uri>
 </heightmap>

Additional context

• Relates to https://github.com/gazebosim/gz-common/issues/596