The open source OpenXR runtime

cmake,external: Add xrt-external-bigceres target master

This depends on a system install of Ceres which contains the EIGENSPARSE feature, which is used downstream for solving large bundle adjustment problems. Signed-off-by: Beyley Cardellio <ep1cm1n10n123@gmail.com> Part-of: <https://gitlab.freedesktop.org/monado/monado/-/merge_requests/2963>