Invited chapter for the edited book New Frontiers in Gravitational Collapse and Spacetime Singularities (Eds. P. Joshi and D. Malafarina, Springer Singapore) *Series title: *Springer Series in Astrophysics and Cosmology *Book title:* New Frontiers in Gravitational Collapse and Spacetime Singularities https://link.springer.com/book/9789819711710