Handle subscribe in resolverMap

[cheapsteak]

This PR adds a handler and typing for resolvers for Subscriptions

As Jesse helped find in graphql-tools, resolve and subscribe are not mutually exclusive

I believe this relates to apollographql/apollo-server#2149

TODO:

• Update CHANGELOG.md* with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

[sebas5384]

hi @cheapsteak and @jbaxleyiii !

I'm building an experimental lib as an alternative to @graphql-modules/core called graphql-modules-fn using the buildServiceDefinition() from the apollo-tools like apollo-server does, and I was wondering if there's plans to merge this PR in order to get subscriptions to work with "modules".

thanks!