Stitcher interstitials should be a discriminated union

[matvp91]

Is your feature request related to a problem? Please describe.

{
  "time:" DateTime;
  "uri"?: string;
  "vastUrl"?: string;
}

Describe the solution you'd like

{
  "type": "vast";
  "time:" DateTime;
  "url": string;
}

{
  "type": "asset";
  "time:" DateTime;
  "uri": string;
  "type"?: InterstitialAssetType;
}