Extend webhook to support bumpers

[birme]

We want to be able to support inserting bumpers that are not treated as an ad break. Depending on the feature described here Eyevinn/lambda-stitch#18 we need to construct the stitch-payload accordingly.

Extend the webhook interface:

export interface WebHookNextVodResponse {
  id: string;
  title: string;
  hlsUrl: string;
  prerollUrl?: string;
  prerollDurationMs?: number;
  bumperUrl?: string;
  bumperDurationMs?: number;
  desiredOffsetMs?: number;
  desiredDurationMs?: number;
}

This is required to resolve the feature request in EyevinnOSC/community#30