libcephfs_proxy: implement client side support for embedded perms

libcephfs_proxy: implement client side support for embedded perms

Implement the code to handle embedded perms or not depending on a
feature flag negotiated during connection.

If embedded permissions are enabled, ceph_userperm_new() will allocate a
local structure with the provided credentials instead of sending it to
the server. ceph_userperm_destroy() will just destroy the allocated
structure. If it's disabled, these function will work as any other
function, sending the request to the server.

Signed-off-by: Xavi Hernandez <xhernandez@gmail.com>
(cherry picked from commit 68505c3e3a3f7c00bcc6a0b2e90932bf52d09575)