lan: add ability to set static leases
this MAC address will get this IP, etc
Note that the backend part is supposed to be done
https://gitlab.labs.nic.cz/turris/foris-controller/foris-controller/blob/master/foris_controller_modules/lan/schema/lan.json#L185
### Steps in this issue
- [x] create endpoint (!259 @fhron)
- [ ] implement UI (This ticket)
## Schema
```json
"dhcp_client_set": {
"type": "object",
"properties": {
"mac": {"type": "string", "format": "macaddress"},
"ip": {
"oneOf": [
{"type": "string", "format": "ipv4"},
{"enum": ["ignore"]}
]
},
"hostname": {"type": "string"}
},
"additionalProperties": false,
"required": ["ip", "mac", "hostname"]
}
```
## enpoint
```<API>/api/lan/set_client```
# Mock
issue