Implement introspection schema.json generation

[theduke]

Implement a method to easily create a schema.json introspection schema used by tools like apollo cli and https://github.com/graphql-rust/graphql-client.

For simplicity I'd just execute the introspection query against the schema and return the result.

[theduke]

Example query:

query IntrospectionQuery {

  __schema {

    queryType {

      name

    }

    mutationType {

      name
    

    }

    subscriptionType {

      name

    }

    types {

      ...FullType

    }

    directives {

      name

      description

      locations

      args {

        ...InputValue

      }

    }

  }

}

fragment FullType on __Type {

  kind

  name

  description

  fields(includeDeprecated: true) {

    name

    description

    args {

      ...InputValue

    }

    type {

      ...TypeRef

    }

    isDeprecated

    deprecationReason

  }

  inputFields {

    ...InputValue

  }

  interfaces {

    ...TypeRef

  }

  enumValues(includeDeprecated: true) {

    name

    description

    isDeprecated

    deprecationReason

  }

  possibleTypes {

    ...TypeRef

  }

}

fragment InputValue on __InputValue {

  name

  description

  type {

    ...TypeRef

  }

  defaultValue

}

fragment TypeRef on __Type {

  kind

  name

  ofType {

    kind

    name

    ofType {

      kind

      name

      ofType {

        kind

        name

        ofType {

          kind

          name

          ofType {

            kind

            name

            ofType {

              kind

              name

              ofType {

                kind

                name

              }

            }

          }

        }

      }

    }

  }

}

[mrceperka]

Juniper is able to process this query, but apollo fails to generate schema.json.
I've received this error:

 Expected a value of type "__DirectiveLocation" but received: "INLINE_SPREAD"

I was looking at https://github.com/facebook/graphql/blob/master/spec/Section%204%20--%20Introspection.md and there FRAGMENT_SPREAD. Maybe there is a typo in juniper...

I see that this is fixed (c7d1481). So I will probably have to wait for another realease or use master.

[theduke]

@mrceperka yeah it's currently in master, so you could use a Git dependency for now until we push out a patch release.

[LegNeato]

I have implemented this. Just cleaning up the code and adding tests before putting up a PR.

[LegNeato]

Hmmm, actually my PR may does fix this issue...it doesn't output in JSON. Which is different.