Implement Cisco-NSO nonstandard parsing

[amtelekom]

Cisco uses some YANG constructs that are not strictly compliant with RFC7950 in their YANG-Files describing Cisco's Network Service Operators built-in functionality,
most notably input and output statements without a body,
and using XPath deref in leafref paths.

This MR enables parsing these constructs when enabling a new crate feature cisco-nso-extensions.