Expose the RA tunnel callbacks in the svn_client API.

* subversion/include/svn_client.h (svn_client_ctx_t):
   New members open_tunnel_func and close_tunnel_func.
* subversion/libsvn_client/ra.c (svn_client__open_ra_session_internal):
   Set the tunnel callbacks in the RA callbacks table.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1531612 13f79535-47bb-0310-9956-ffa450edef68