Update TrailType with field masterImage for Galleries

The TrailType type doesn't have the field `masterImage` while the FETrailType type has.

In [`getSourceImageUrl_temp_fix`](https://github.com/guardian/dotcom-rendering/blob/main/dotcom-rendering/src/lib/getSourceImageUrl_temp_fix.ts#L22) we are getting the image url (i.guim) rather than the masterImage url (media.guim) and converting it into a media.guim url.

Then later on, in [`generateImageURL`](https://github.com/guardian/dotcom-rendering/blob/main/dotcom-rendering/src/lib/image.ts#L49) the media.guim url is converted back to i.guim url :)

We'd probably want to use the masterImage rather than the image url from FETrailType and avoid the multiple conversions.