Asynchronous HTTP 202 + user cancellation

[Jason-DNetto]

Instantiating the cesium Request object in TerriaJsonCatalogItem and passing it down through to cesium Resource objects, so that the user clicking 'Remove' on the ResultPendingCatalogItem cancels the Request in the request object and stops creating xhr calls.

[kring]

Thanks @Jason-DNetto! Can I bug you to make this a PR into the cesium-1.46 branch? Much easier to review that way.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ kring
❌ Jason-DNetto
_{You have signed the CLA already but the status is still pending? Let us recheck it.}