type_caster_generic: add set_foreign_holder method for subclasses to implement (#5862)

• type_caster_generic: add set_foreign_holder method for subclasses to implement

• style: pre-commit fixes

• Rename try_shared_from_this -> set_via_shared_from_this to avoid confusion against try_get_shared_from_this

• Add comment explaining the limits of the test

• CI

• style: pre-commit fixes

• Fixes from code review

• style: pre-commit fixes

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>