Update ApolloServer's typeDefs types to more accurately reflect r…

…eality.

While no TypeScript user would have ever been able to use it as such, Apollo
Server has for a long time (as a side-effect of its past reliance on
`graphql-tools`) accepted string types (and arrays of string types) for
`typeDefs`.

This hasn't been reflected in the TypeScript types, and has thus made it
possible to sandbag non-TypeScript users (which let's remind ourselves, is
still the majority) who were strings, when new code — like that in apollographql#2762 —
didn't defend against what TypeScript indicated would be fine.

Even if we don't necessarily support string types as a common pattern
anymore, they still work, and we need to make sure we're coding for what we
expect to work until the time comes to make a breaking change.

packages/apollo-server-core/src/types.ts

@@ -68,7 +68,7 @@ type BaseConfig = Pick<
 // fields that are not specific to a single integration
 export interface Config extends BaseConfig {
   modules?: GraphQLSchemaModule[];
-  typeDefs?: DocumentNode | Array<DocumentNode>;
+  typeDefs?: DocumentNode | Array<DocumentNode> | string | Array<string>;
   parseOptions?: GraphQLParseOptions;
   resolvers?: IResolvers;
   schema?: GraphQLSchema;