vfs_ceph_new: Dynamically open library for 'proxy' mode

vfs_ceph_new: Dynamically open library for 'proxy' mode

Use dlopen() to load either of the shared libraries(libcephfs.so or
libcephfs_proxy.so) based on the configuration for 'proxy' module
parameter. Further down the line we will define the required APIs
as function pointers within the config structure.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15703

Pair-Programmed-With: Anoop C S <anoopcs@samba.org>
Signed-off-by: Guenther Deschner <gd@samba.org>
Signed-off-by: Anoop C S <anoopcs@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>