Add the ability to use a custom backend.

[dkruk]

From my point of view, the backend is a good place to add some protection(e.g. query depth or complexity). In the custom backend it possible to analyze document AST and raise an exception, if the query does not comply with the rules before it will be executed.

graphql_server.execute_graphql_request accepts the parameter backend and if it None get default backend.

[wumpus]

Apparently a custom backend is also the preferred way to add caching of parsed queries, as per graphql-python/graphql-server#7 (see also #2)

Since the desire to have cached parsing is popular, it would be nice to have a worked-out example of how to do that in our docs.

[Cito]

Thank you. This should be possible with #10 already.