swig: replace libnvmf_context with dict-based ctrl constructor

The libnvmf_context object was a leaky abstraction: Python callers had to
create it, configure it, pass it to ctrl(), then discard it — despite never
needing it after construction. This commit restores the ergonomics that were
lost when the C API switched from libnvme_fabrics_config to libnvmf_context.

The public header declares libnvmf_context as an opaque forward declaration,
which caused SWIG to treat it as an opaque pointer type and override any
user-defined %typemap. Fix this by providing an empty struct body in the SWIG
parsed section, which gives SWIG a complete type and restores normal typemap
precedence without modifying any C header.

The %typemap(in) / %typemap(freearg) pair for struct libnvmf_context * now
handles the full fctx lifecycle transparently: the typemap creates the fctx
from the Python dict (using arg1 as the global context), the constructor body
calls libnvmf_create_ctrl(), and freearg frees the fctx. The typemap is
reusable for any future function that takes struct libnvmf_context *.

The dict supports all configurable fields: connection identity (subsysnqn,
transport, traddr, trsvcid, host_traddr, host_iface), the full
libnvme_fabrics_config (via libnvme_fabrics_config_set_* accessors and
libnvmf_context_get_fabrics_config), host identity (hostnqn, hostid), crypto
and TLS (hostkey, ctrlkey, keyring, tls_key, tls_key_identity), and
persistence. All context-level fields are set via their public API setters.

An autodoc string on the constructor documents every supported key so that
help(nvme.ctrl) is the complete reference. Tests are updated to use the new
dict-based API.

Signed-off-by: Martin Belanger <[email protected]>
Assisted-by: Claude Sonnet 4.6 <[email protected]>

Author: Martin Belanger