api: create scan policies

[scriptnull]

POST /api/v1/policies

Expected body

{
    "name": "<>", // required
    "trigger": {
        "type": "<>" // required. Could be one of "cron" or "ci"
     }
}

If type is cron, expect a schedule string and a non-empty image section. Refer those sections in #32 for more details.