As discussed here [1], we want a way to mark runtimes to be kept even
when they are unused by any apps and we are removing such runtimes.
Currently this is a command that can be run manually; a subsequent
commit will pin runtimes automatically if they are installed
independently of any app.
A unit test is included.
[1] https://github.com/flatpak/flatpak/issues/2639#issuecomment-662311756
d2d5397cc1
