Rename the config options for the RPC mechanism as GDAL_API_PROXY, GDAL_API_PROXY_SERVER and GDAL_API_PROXY_CONN_POOL; document the mechanism (#4979)

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@25606 f0d54148-0727-0410-94bb-9a71ac55c965