Add support for Dictionary/Hashmap. See https://github.com/realm/realm-java/issues/759 for the initial discussion.

Possible API:

// New class
// See API for RealmResults/RealmList for which extra methods we need
class RealmSet<V> : Set<V> { ... }

// new change listener. Should it be different than the one for Maps?
class UnorderedCollectionChangeset {
  // Figure out API
}

// Example Usage
open class Person : RealmObject() {
  var set: RealmSet<Dog> = RealmSet()
}
val person = realm.createObject<Person>()
person.set.addChangelistener { set, changeset ->
  // Handle changes to set
}

TODO (probably not complete):

• [ ] Determine possible API. Ideally, we should be able to store all supported Realm types as both keys and values
• [ ] New callback type: UnorderedCollectionChangeset. Determine API.
• [ ] Add support for RealmResults.setSet(set)
• [ ] Add support for DynamicRealmObject.setSet(set)/DynamicRealmObject.getSet().
• [ ] Add support in RealmFieldType
• [ ] Add support in RealmObjectSchema
• [ ] Add support in RealmQuery. Which types of queries should be supported? containsValue ?
• [ ] copyToRealm
• [ ] copyToRealmOrUpdate
• [ ] insert
• [ ] insertOrUpdate
• [ ] copyFromRealm
• [ ] How should sets be supported in JSON? Like Arrays?
• [ ] createObjectFromJson(Class, InputStream)
• [ ] createObjectFromJson(Class, JSONObject)
• [ ] createObjectFromJson(Class, String)
• [ ] createOrUpdateObjectFromJson(Class, InputStream)
• [ ] createOrUpdateObjectFromJson(Class, JSONObject)
• [ ] createOrUpdateObjectFromJson(Class, String)
• [ ] createAllFromJson(Class, InputStream)
• [ ] createAllFromJson(Class, JSONArray)
• [ ] createAllFromJson(Class, String)
• [ ] createOrUpdateAllFromJson(Class, InputStream)
• [ ] createOrUpdateAllFromJson(Class, JSONArray)
• [ ] createOrUpdateAllFromJson(Class, String)

See https://github.com/realm/realm-java/pull/6722 for where changes need to happen

Blocked by https://github.com/realm/realm-core/issues/3717