cast: Add is_generic_type<T>

[EricCousineau-TRI]

Description

Adds explicit way to denote a is_generic_type<>, which is effectively bullet point (1) from the docs: https://pybind11.readthedocs.io/en/stable/advanced/cast/index.html

Permits distinction from bullet points (2) and (3)

Suggested changelog entry:

Adds is_generic_type<> trait