Support for extensions

In `graphene-tornado`, we've added support for [extensions](https://github.com/graphql-python/graphene-tornado#extensions) similar to Apollo Server. These extensions provide the capability to do things like add [Apollo Tracing](https://github.com/graphql-python/graphene-tornado/tree/graphql-extensions-tracing) or distributed tracing to requests.

This capability would be great to add to `graphql-server-core` (it's a blocker for `graphene-tornado` to migrate). Is there interest in having this functionality in this project? Specifically, I'm suggesting we add the extension points defined [here](https://github.com/graphql-python/graphene-tornado/blob/graphql-extensions-tracing/graphene_tornado/extension_stack.py#L13).