Merge Unity.Abstractions into Container

# End of life for [Unity.Abstractions](https://github.com/unitycontainer/abstractions)

As described in [unity#347](https://github.com/unitycontainer/unity/issues/347) package [Unity.Abstractions](https://github.com/unitycontainer/abstractions) will no longer be developed. Entire code base for the container will be maintained in this repository.

