Add the ability to select child assets for property value graphing

[wz2b]

Is your feature request related to a problem? Please describe.
Sort of. When I use Get Property Value History, the 'Asset' dropdown menu ONLY shows assets at the top level. In my case, a top-level asset is like a "Plant", underneath that are "Lines", under that are Machines; and each machine can have something like a programmable logic controller underneath it. The asset hierarchy follows this same pattern. The SiteWise datasource only shows the top-level

To resolve this today, what you have to do is either query the graphdb or log into the AWS SiteWise console and figure out the GUID of the asset that owns the sensor (underneath that machine, line, and plant). Then you can paste that GUID into Grafana and it will give you a drop-down list of the graphable properties. That's an okay way to do it but it requires manual, external steps.

Describe the solution you'd like
I think that the "Asset" selection needs to be more like a nested list. It could still be implemented as a single drop-down, perhaps, with the sub-assets indented, but it might not be that easy to make that work with real-time type-to-search in that dropdown. If that doesn't work, I think you would need to turn that UI element into some kind of tree selector (which is something I have yearned for as a plugin developer).

Describe alternatives you've considered
You can still select assets that aren't at the top level, but you have to know their GUID. This makes the data accessible, but it's not very friendly to have to go and locate the child asset GUID.

[wz2b]

This was a stupid feature request. There is a button named "Explore" that you can push and it descends through the hierarchy. I just had a mental blind spot for that button for some reason. Sorry for the noise.