Improve create alerts input

[erenJag]

• Here we check in each alert decisions if start_ip & end_ip are valid. Before merging that we need to evaluate if it's less efficient or not.

Example result :

eren@linux ~/crowdsec (improve_createAlerts_input)$ http POST localhost:8080/v1/alerts 'Authorization: Bearer eyJhbGciOiJI.....' < pkg/apiserver/tests/alert_sample.json 
HTTP/1.1 400 Bad Request
Content-Length: 66
Content-Type: application/json; charset=utf-8
Date: Tue, 01 Dec 2020 11:32:08 GMT

{
    "message": "'start_ip' must be lower than 'end_ip' in a decision"
}

[buixor]

lgtm