Support Hapi
Hi!

As you may know Hapi has built in parsing of multipart requests and there are currently some issues with combining graphql-upload with that. If you want more background, check out this PR against apollo-server-hapi https://github.com/apollographql/apollo-server/pull/4943

The nicest way to integrate Hapi would be to let Hapi parse the request and provide an API from graphql-upload to handle that case, which you also describe in this old comment https://github.com/jaydenseric/graphql-upload/issues/5#issuecomment-309706215

In this comment https://github.com/apollographql/apollo-server/pull/4943#issuecomment-796738593 in the above PR you can see how I do this with Hapi, but with a lot of copy and paste from `processRequest`. Are you interested to further explore an API that could be exported for Hapi users as well used internally by `processRequest` to avoid this duplication?

