Update type definitions to expose `AppliedDirective`s. **This initial implementation only exposes applied directive as metadata and does not modify runtime wiring to process them**. Exposing directives is a pre-req to enable support for [Apollo Federation](https://www.apollographql.com/docs/federation/) (related: graphql-go#492).

Following the pattern from [`graphql-java`](https://github.com/graphql-java/graphql-java) and [`HotChocolate`](https://github.com/ChilliCream/hotchocolate) where they make distinction between directive type definition (i.e. `directive @foo on FIELD_DEFINITION`, maps to existing `Directive struct` for backwards compatibility) and where directives are actually applied within the schema (i.e. `type Foo @bar { baz @Qux }`, maps to new `AppliedDirective` struct`).