Feature request: add support for sway input events

Subj. I tried to do it myself (and make a pull request), but it looks like I don't fully understand how i3ipc-python handles events, so it didn't work. What I want is to be able to subscribe to `"input"` event, which is issued for example when the keyboard layout is switched. It was added in sway-1.2. `swaymsg` command for reference:
```
swaymsg --type subscribe --monitor --raw  '["input"]'
```