make svn_client__open_ra_session() public

See the following mailing list thread:
http://subversion.tigris.org/servlets/BrowseList?list=dev&by=thread&from=320439

In short:
the currently private function svn_client__open_ra_session() should be made a
public funtion. Clients will have it much easier to get a ra-session with that
function instead of using ra_open(), because svn_client__open_ra_session() takes
a svn_client_ctx_t structure.