Investigate how to publish to Unity Asset Store #723

Feature

Previously we did not publish Cesium for Unity to the Asset Store due to incompatibility with the terms of service. However, the terms have since been updated and now allow us to publish.

We should look into how we can make both Cesium for Unity and the samples project officially available on the Asset Store. We would need to:

• Create a "CesiumUnity" account (we're all using Unity through individual work accounts; we shouldn't publish the package as ourselves).
• Publish the package + samples project as assets under "CesiumUnity"
  • Need to setup store pages (but probably can just copy Unreal's and modify)
  • Need to figure out release process
  • Could potentially involve needing to sign our packages (Sign our release packages #659)

While we introduce this, we will continue to support the scoped package registry method for installing Cesium for Unity, but let users know they should migrate.