pybind/rados: enable construction with CephContext
The CephContext pointer is passed around inside
a Python capsule. It would have initially been
created by some other (non-cython) bit of C++
code, such as ceph-mgr.
This enables mgr modules to instantiate librados
with all the same config/auth/id as the mgr daemon
that they are running within. Useful if you
want to spin up e.g. a librbd instance.