Adapt CFFI bindings for libyang v5 (soversion 5.0.0+)

Update cdefs.h, source.c, and Python call sites to match the libyang
4.2.2 -> 5.0.0 API changes:
- parent field type changed from lyd_node_inner* to lyd_node* across
  all node structs
- lyd_any_value_str gained a LYD_FORMAT parameter
- lys_find_child signature was reworked
- lyd_node_any lost the union lyd_any_value/LYD_ANYDATA_VALUETYPE fields
- LYD_NEW_VAL_BIN / LYS_GETNEXT_WITHSCHEMAMOUNT removed

tests: fix failures caused by libyang v5 changes

Add missing ietf-inet-types YANG module to the test search path, as
libyang v5 no longer resolves it automatically. Update the module data
callback test to handle the ietf-inet-types dependency, and adjust
expected error paths to match libyang v5 output.

Signed-off-by: Antonio Prcela <antonio.prcela@gmail.com>