Add example dynamic subscription books

[gianalarcon]

Add a fully dynamic schema example. It uses query, mutation and subscription with a dynamic schema in Poem framework. This example is the dynamic version of https://github.com/async-graphql/examples/tree/master/poem/subscription

[sunli829]

Thanks!😁

[gianalarcon]

Thanks for your approval. All the projects that your team is building is really amazing. 辛苦了大家 加油！