Make repeated fields and field order of object and input extensions match graphql-java in 15.0.0 #865

Interface extensions now keep the first of a repeated field and log a warning (#855), but object extensions still silently keep the last one:

type Query {
    "from the type"
    greeting: String
}

extend type Query {
    "from the extension"
    greeting: String
}

builds greeting with "from the extension". Input objects also list the fields of their extensions before their own, so input Filter { term: String } plus extend input Filter { limit: Int } gives limit, term.

graphql-java rejects the repeated field and lists base fields first. Changing either changes existing schemas, so this has to wait for 15.0.0, together with #863.