Swap mergeSchemas with makeExecutableSchema

[RobertBroersma]

Basically the title says it all!

Motivation in #333

Unfortunately what @peterp mentioned in #333 (comment) didn't fly; just swapping out apollo-server-lambda for graphql-tools-fork didn't work. It seems like graph-tools-fork is improving on the seemingly abandoned graphql-tools, but not (yet) on custom directive support for mergeSchemas.

What does however support custom directives is makeExecutableSchema. In order to use this I needed to merge the typeDefs, and the only/quickest/cleanest way I could find to do this is using merge-graphql-schemas, the second package mentioned by @peterp.

Using this PR you can pass schemaDirectives in makeMergedSchemas like so:

schema: makeMergedSchema({
  schemas,
  services: makeServices({ services }),
  schemaDirectives: {
    auth: AuthDirective,
  },
}),

[peterp]

This is awesome, thank you!