Export 'ExpressContext' out of the main module.

Can we get the ExpressContext type exported out of the main module? Currently it’s available through the ApolloServer submodule but i prefer not having imports from submodules.

[eliw00d]

@abernix Here is another example of usage that I came across, if it helps:

import { ExpressContext } from 'apollo-server-express/src/ApolloServer'

const gateway = new ApolloGateway({
  buildService: ({ url }) => new RemoteGraphQLDataSource<ExpressContext>({
    ...
  })
})

Would be nice in this case not to drill into src in the import.

[shellscape]

Thanks!