Provide stable mechanism for discovering `nethost`

### Is your feature request related to a problem? Please describe.
The `nethost` assets are defined in the AppHost package. Tools like [DNNE](https://github.com/AaronRobinsonMSFT/DNNE) need to come up with clever ways to discover where they are located. On Windows, they may be located in the SDK install. On Linux, they may need to be downloaded. There is no simple way to ask - "Where is the AppHost package root?"

### Describe the solution you'd like

`TargetAppHostPackageRoot` property.

### Additional context

I solved this in https://github.com/AaronRobinsonMSFT/DNNE/pull/209. The gist is setting a property to ensure the AppHost package is resolved and then looking at a specific item in a target that is dependent on two different targets.

```xml

[RID to target]

%(ResolvedAppHostPack.PackageDirectory)/runtimes/[RID to target]/native
```