One can boot into the command from grub by editing kernel parameters.
Another way is edition the grub configuration and setting GRUB_CMDLINE_LINUX_DEFAULT="text". But now it's not possible to boot into a graphical env.
So is there way to create menu entry just for command line so it will be one of the ways to log into the system?
It might be possible to boot into a bootable image from grub so you don't need to set up another bootable partition.
Or you could disable your display manager in systemd. This will start in console, then if you want X just run startx.
Or you could change your display manager to Lemur, which supports X, Wayland, and TTY sessions.
Or you could just press control-alt-F2 at the login screen to switch to a console.