API_PROXY: make server declare the instruction it can process so that the client can default to the local PAM implementation if needed; add a toy Python API_PROXY server that redirects on GDAL implementation, but could be used to implement a Python GDAL driver.

git-svn-id: https://svn.osgeo.org/gdal/trunk@25644 f0d54148-0727-0410-94bb-9a71ac55c965