Support for Custom libinput acceleration profiles

[BrendonJL]

1. Recently started using a trackball and found that custom mouse acceleration curves would be very useful
2. Would like the abilIty to set acceleration to custom instead of flat or adaptive to take advantage of the libinput_device_config_accel_set_points() available in libinput 1.26+
3. Found something simlar Specific input device configuration #371 but it was about per-device config which was related but this is more about what settings are available rather than which device.

P.S. This was written by me a human but I used claude to research what was available in libinput and whether similar items had been brought up for Niri before. This is one of my first times trying to make a suggestion like this so if there are any issues on formatting, presentation, etc I apologize!