[Smart holder] Support void pointer capsules

[wangxf123456]

Description

Make smart holder type casters support void pointer capsules.

The original motivation for this is to support passing SWIG objects to pybind11 wrapped functions. This is achieved by checking whether the object has a particular method defined. Suppose we have the following C++ code:

namespace pybind11_tests {
namespace void_ptr_capsule {
class Obj { };

void func(const Obj* obj) { }
}
}

Then if an object has the method as_pybind11_tests_void_ptr_capsule_Obj defined, and the method returns a capsule with name pybind11_tests::void_ptr_capsule::Obj, then we think this object can be converted to pybind11_tests::void_ptr_capsule::Obj, so we accept the object as a parameter.

[rwgk]

LGTM — We're running global testing overnight to be on the safe side.

[Skylion007]

Very tiny nit, this could be an allocation of "as_" and += type_name and remove an allocation.

[rwgk]

This PR was globally tested. Merging now. (Skipping the CI rerun awaiting approval: very low risk.)