Add Placement Group Server Action

## Affects:
Server Actions

## Need for this:
Links the placement group server action API

## Hetzner API documentation
https://docs.hetzner.cloud/#server-actions-add-a-server-to-a-placement-group

## Tasks
- [x] Implement method on Server class
- [x] Add new Action enum
- [x] Implement change in ServerChangeListener
- [x] Implement change in ServerService
- [x] Add testing