feat(shared state server): added shared state server

[sr258]

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[sr258]

@JPSchellenberg This is my first draft of a shared state server.

There are two sample content types:
https://github.com/sr258/h5p-multiuser-quiz (Kahoot-style quiz)
https://github.com/sr258/h5p-sharedb-test (reaction vote)

The shared state server can be used in the rest example by starting it with npm run start:shared-state. You need to add one of the example content types for it to work.

The root of the h5p-shared-state-server contains a documentation on the architecture and how to develop with it.

There's also a WordPress extension (via microservice) that uses the shared state server: https://github.com/sr258/wp-h5p-shared-state

I'd like to merge and release it relatively soon so that I can publish a first draft and request feedback by the community.