Add attach a server to network action

## Affects:
Server Actions

## Need for this:
Links the attach a server to network action API

## Hetzner API documentation
https://docs.hetzner.cloud/#server-actions-attach-a-server-to-a-network

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