Expose `WaybackMetadata` from `getWaybackItemsWithLocalChanges` so one can get imagery date rather than release date

ESRI has a concept of
- release/layer date (mosaic/collage date of the processing),
- vs imagery acquisition date.

`Esri/wayback-core` similarly has a
- [getMetadata](https://github.com/Esri/wayback-core#getmetadata) api that returns imagery-date at a given xyz location + release number,
- vs the [getWaybackItemsWithLocalChanges](https://github.com/Esri/wayback-core#getwaybackitemswithlocalchanges) that returns only release/layer date.

It would be useful for `getWaybackItemsWithLocalChanges`, since it is passed the xyz of the location where one want to know what release layer has local changes, to expose the underlying imagery date. Current return type of the promise is expose `Promise<`[WaybackItem](https://github.com/Esri/wayback-core#waybackitem)`[]>`. It could be nice to append `WaybackMetadata`. To allow backward compatibility, this could be exposed via a flag argument so the return structure stays the same.

Start by inspecting getWaybackItemsWithLocalChanges, its WaybackItem return type, and the getMetadata API that provides imagery dates. Determine how WaybackMetadata can be exposed without breaking the existing return structure; done means callers can obtain imagery dates for changed locations while existing usage remains compatible.

Written by the indexing model from the issue text.