Add package manifest for pulling HEU through UPM

[tlaedre]

This change allows HEU to be added to a project as a package instead of a folder hierarchy manually copied inside Assets/.

A side effect of the change is that the plugin can now be installed and updated directly from Git(Hub) without requiring any manual steps - this is especially useful for teams that fork and modify HEU as it can be pulled directly from their source control (or from a scoped package registry if that's preferred).

IMHO this is the preferred way of adding middleware/plugins to Unity projects as it keep the Assets/ folder clean from external tools and reserves it exclusively for project content.