Add libglvnd packages/Add EGL support (#14572)

* add new package: "libglvnd-frontend"

* add +glvnd variant to opengl package

* add +glvnd variant to mesa package

* add +egl variant to paraview package

* add libglvnd-frontend entries to default packages config

* fix style

* add default providers for glvnd virtuals

add default providers for glvnd-gl, glvnd-glx, and glvnd-egl

* WIP: rough start to external OpenGL documentation

* rename libglvnd-frontend package and backend virtual dependencies

* update documentation

* fix ligvnd-be-* typos

* fix libglvnd-fe package class name

* fix doc parse error