IHubContent should have metadata, and provide a way to fetch it

[tomwayson]

follow up from #330 and related to #322 (but more limited in scope).

As I indicated in #322 (comment), I think getContent() should return metadata when it exists (i.e. when fetching from the Hub API). I'm not sure that it should proactively make an async request to get it otherwise. We can start by adding something like getContentMetadata() that would fetch it from wherever the enrichment currently fetches it from and the client can call that as needed.

[drewdaemon]

@tomwayson This may be helpful: https://www.arcgis.com/sharing/rest/content/items/d9ea4896eb7b4471b6839771c51de091/info/metadata/metadata.xml

[drewdaemon]

Even better: https://www.arcgis.com/sharing/rest/content/items/1ce4060e854747038b153c5c8f8c894d/info/metadata/metadata.xml

[ajturner]

This is a good plan. However, please be aware that the purpose of getContent() and other Hub.js functions is to provide a clear, consistent interface to the Hub models. it's not just a wrapper for promise chains :)

Specifically, in the near future item editors will be able to configure their metadata display. By far the most common request is to choose what shows in "Last Updated" because it's often not the "ArcGIS Item edited date" but instead some real-world date when a surveyor went into the field. Therefore, the Content.publishedDate may be coming from Content.publishedDateSource = 'metadata.dataIdInfo.idCitation.date.pubDate' such as from DC Trees.

So the getContent() call will look at it's display configuration (likely in either item.properties or another sidecar in resources) to know it should fetch this information and merge specific results into the core content model. @tomwayson @drewctate @mikeringrose

[tomwayson]

Looks like the hub-indexer uses https://www.npmjs.com/package/fast-xml-parser to parse the metadata XML response, and it looks like we can use that in a browser and it's only 5.5kB gzipped, so I'll plan to implement this using that library. The parser is configured here.

[tomwayson]

relies on Esri/arcgis-rest-js#738