Adding virtual_overrider_self_life_support.

[rwgk]

Enables safely passing unique_ptr from Python to C++, even for trampolines.

A disowned trampoline Python object is kept it alive until the unique_ptr pointee is destroyed. The critical small trick needed was found in PyCLIF (probably invented by @mrovner):

google/clif@acacae6

The critical part of the trick is: the Py_INCREF is triggered only when the trampoline Python object is in the process of being disowned.

This PR goes beyond PyCLIF, in that the disowned Python object still maintains a pointer that is only reset and deregistered when the unique_ptr pointee is deleted, which means methods of the pointee can still be safely accessed from Python. This could be viewed as "lazy disowning".

[rwgk]

The CI is green, also with PYBIND11_USE_SMART_HOLDER_AS_DEFAULT (#2879). Google-internal testing is also green. Merging here to unblock follow-on PyCLIF work. This change will get reviewed internally. I'll open a new PR if there are requests for changes.