Support `"file_access": "access_denied"` in SlackFile model

[zmarushchak-hs]

Currently we fail to parse Slack response like below. This PR adds custom deserialiser to handle responses like this.

{
      "id" : "F05RFJX6KL4",
      "file_access" : "access_denied",
      "created" : 0,
      "timestamp" : 0,
      "user" : "U01K1BQFK3R",
      "filetype" : "png"
    }