Add endpoints for static DHCP leases

Overview

Add following endpoints:

• /lan/update_client - update DHCP client config
• /lan/delete_client - delete DHCP client

The difference between set_client and update_client is that set_client should fail to add DHCP client which already exists. I.e. don't accidentally overwrite existing config when setting new static lease.

update_client should fail to update non-existing client.

delete_client should fail on deleting non-existing client.

Input data

ip, mac and hostname has to be unique, unless you are overwriting the same host - e.g. update just IP address of dhcp client, but keep the same hostname

• set_client + update_client

{
    "type": "object",
    "properties": {
        "mac": {"type": "string", "format": "macaddress"},
        "ip": {
    	"oneOf": [
    	    {"type": "string", "format": "ipv4"},
    	    {"enum": ["ignore"]}
    	]
        },
        "hostname": {"type": "string", "minLength": 1}
    },
    "additionalProperties": false,
    "required": ["ip", "mac", "hostname"]
}

• delete_client

{
    "type": "object",
    "properties": {
        "mac": {"type": "string", "format": "macaddress"}
    },
    "additionalProperties": false,
    "required": ["mac"]
}

Foris-controller return either {'result': True} on success or {'result': False, 'reason': 'reason-what-went-wrong'} on failure.

Reasons for failure for particular operations are defined in json schema (set/update/dhcp_client_{set,update,delete}_reply):

https://gitlab.nic.cz/turris/foris-controller/foris-controller/-/blob/214-dhcp-allow-managing-of-static-leases/foris_controller_modules/lan/schema/lan.json#L134-208

return values

But we don't handle these failures in flask yet, because we would need to refactor error handling in reforis and re-test whole reforis again.

So I propose two approaches

• either endpoints will return either {'result': True} or string with error message (e.g. Cannot add lease). It won't be the great UX at the moment, but we could add better error handling as separate MR.
• or return either {'result': True} on success or {'result': False, 'reason': 'reason-what-went-wrong'} on failure and handle that failure in JS - effectively bypassing API errors

cc: @msasek, @agumroian

Part of #168 (closed)