Value classes can be public

These were originally private to prevent prefixes from leaking. We've
switched to handling prefixes in application config, so these can be
publicly accessible now.

src/main/scala/fable/GroupId.scala

@@ -6,7 +6,7 @@ import pureconfig.generic.semiauto.deriveReader
 /**
   * Value class for consumer group IDs.
   */
-case class GroupId private (name: String) extends AnyVal
+case class GroupId(name: String) extends AnyVal
 
 object GroupId {
   implicit val groupIdConfigReader: ConfigReader[GroupId] = deriveReader

src/main/scala/fable/Topic.scala

@@ -6,10 +6,8 @@ import pureconfig.generic.semiauto.deriveReader
 /**
   * Value class for topic names.
   */
-case class Topic private (name: String) extends AnyVal
+case class Topic(name: String) extends AnyVal
 
 object Topic {
-  private[fable] def apply(name: String): Topic = new Topic(name)
-
   implicit val topicConfigReader: ConfigReader[Topic] = deriveReader
 }