Export supergraph as SDL

[gmac]

⚠️ Minor breaking change

This replaces the supergraph.export and Supergraph.from_export methods with supergraph.to_definition and Supergraph.from_definition.

Where the old export methods would return/receive multiple export artifacts (SDL + JSON), this new serialization/deserialization process writes everything as one annotated SDL file, ex:

directive @resolver(
  arg: String!, 
  federation: Boolean, 
  field: String!, 
  key: String!, 
  list: Boolean, 
  location: String!
) repeatable on INTERFACE | OBJECT | UNION

directive @source(location: String!) repeatable on FIELD_DEFINITION

type Query {
  a(id: ID!): T @source(location: "alpha")
  b(id: ID!): T @source(location: "bravo")
}

type T 
  @resolver(location: "alpha", key: "id", field: "a", arg: "id") 
  @resolver(location: "bravo", key: "id", field: "b", arg: "id") {
  a: String @source(location: "alpha")
  b: String @source(location: "bravo")
  id: ID! @source(location: "alpha") @source(location: "bravo")
}

Migration steps

• See revised documentation.
• Replace supergraph.export with supergraph.to_definition. This now generates a single SDL string with all data rather than multiple artifacts.
• Replace Supergraph.from_export with Supergraph.from_definition, passing in only the composed SDL string and executables map.