[Feature] Make Submodule of main Toolkit project?

## Describe the problem this feature would solve
We should probably add this repo as a submodule to our main toolkit repo to help with integration within the main Sample app.

Currently, we have to build the Toolkit (which is referenced by the graph controls), then build the Graph Controls, and then reference those within the Sample App to build that. This causes problems if underlying types change.

## Describe the solution
We should make the projects within this repo aware of if they're within a sub-module somehow (probably a build property that's only flagged on the Main Repo). This would switch the `PackageReferences` used here to be `ProjectReferences`.

In this way, we could include the sub-module in the main repo and build all the components as part of that CI in addition to the Sample App being unified.

Any changes to the main repo could then be fixed here and then move the sub-module marker and re-tested in the sample app a lot more easily without having to do NuGet shenanigans.

## Describe alternatives you've considered
Otherwise, we'd have to probably move this whole repo within the context of the main repo, but then that makes it harder to just pick up and contribute to the graph related controls.

FYI @azchohfi @shweaver-MSFT thoughts?