Add Room Manager example

[balins]

This PR simplifies (hopefully) and migrates the Room Manager POC that has been used internally by our dev team.

[kamil-stasiak]

Nice job! You've created very simple code. However, I don't understand why you changed Fastify to Express. We had a working solution based on Fastify.

This approach is simpler, but it lacks some features from the previous one, such as:

• Environment variables validation
• Logger
• Meaningful error responses in responses
• OpenAPI schema
• HTTP endpoint parameter and response validation

I thought that we would develop this app as both an Elixir Videoroom replacement and a simple example. For me, code written in Fastify is just as easy to read as one written in Express.

[balins]

The reason behind replacing Fastify with Express was popularity, while I agree Fastify is better for building production-grade apps, Express is a much more known framework, which counts towards simplicity of the example, which was our number 1 objective. As for the missing features, I'm not sure if they are crucial. The purpose of this example was to demonstrate the SDK usage as simply as possible.

Whether to make this example a replacement for the Elixir room manager is TBD. But if we want to do this, I think we should drop the simplicity requirement and make this example more sophisticated (basically, rewind it to your Room Manager POC).