Make the direction of the @relationship directive an enum

[Andy2003]

In the examples I saw that the @relationship directive is used with a string value as the directions' argument.
I would suggest to make this field an enum value, so the user knows the possible values.

directive @relationship(type:String!, direction: RelationDirection = OUT) on FIELD_DEFINITION

enum RelationDirection {
  IN
  OUT
}

[darrellwarde]

Another good idea, but closed for tidiness as we would address this at the same time as addressing neo4j/graphql-tracker-temp#13.