Make Optional.Absent a data object (#6686)

This produces a better `toString()` for logging purposes.

Author: JakeWharton

libraries/apollo-api/api/apollo-api.api

@@ -831,6 +831,9 @@ public abstract class com/apollographql/apollo/api/Optional {
 
 public final class com/apollographql/apollo/api/Optional$Absent : com/apollographql/apollo/api/Optional {
 	public static final field INSTANCE Lcom/apollographql/apollo/api/Optional$Absent;
+	public fun equals (Ljava/lang/Object;)Z
+	public fun hashCode ()I
+	public fun toString ()Ljava/lang/String;
 }
 
 public final class com/apollographql/apollo/api/Optional$Companion {

libraries/apollo-api/api/apollo-api.klib.api

@@ -1322,7 +1322,11 @@ sealed class <#A: out kotlin/Any?> com.apollographql.apollo.api/Optional { // co
         final fun toString(): kotlin/String // com.apollographql.apollo.api/Optional.Present.toString|toString(){}[0]
     }
 
-    final object Absent : com.apollographql.apollo.api/Optional<kotlin/Nothing> // com.apollographql.apollo.api/Optional.Absent|null[0]
+    final object Absent : com.apollographql.apollo.api/Optional<kotlin/Nothing> { // com.apollographql.apollo.api/Optional.Absent|null[0]
+        final fun equals(kotlin/Any?): kotlin/Boolean // com.apollographql.apollo.api/Optional.Absent.equals|equals(kotlin.Any?){}[0]
+        final fun hashCode(): kotlin/Int // com.apollographql.apollo.api/Optional.Absent.hashCode|hashCode(){}[0]
+        final fun toString(): kotlin/String // com.apollographql.apollo.api/Optional.Absent.toString|toString(){}[0]
+    }
 
     final object Companion { // com.apollographql.apollo.api/Optional.Companion|null[0]
         final fun <#A2: kotlin/Any> presentIfNotNull(#A2?): com.apollographql.apollo.api/Optional<#A2> // com.apollographql.apollo.api/Optional.Companion.presentIfNotNull|presentIfNotNull(0:0?){0§<kotlin.Any>}[0]

libraries/apollo-api/src/commonMain/kotlin/com/apollographql/apollo/api/Optional.kt

@@ -29,7 +29,7 @@ sealed class Optional<out V> {
   }
 
   data class Present<V>(val value: V) : Optional<V>()
-  object Absent : Optional<Nothing>()
+  data object Absent : Optional<Nothing>()
 
   companion object {
     @JvmStatic