enrich-pubsub: add reference.conf and provide minimal config example

[benjben]

Take the opportunity to make these configurable :

/**
   * Set the delay threshold to use for batching. After this amount of time has elapsed (counting
   * from the first element added), the elements will be wrapped up in a batch and sent.
   *
   * If the source MaxQueueSize is small, then the queue could block the app by waiting between
   * batches if DelayThreshold is too large.  If the source MaxQueueSize is sufficiently large,
   * then the DelayThreshold should not ever cause blocking.
   */
  val DefaultDelayThreshold: FiniteDuration = 200.milliseconds

  /**
   * A batch of messages will be emitted to PubSub when the batch reaches 1000 messages.
   * We use 1000 because it is the maximum batch size allowed by PubSub.
   * This overrides the permutive library default of `5`
   */
  val DefaultPubsubMaxBatchSize = 1000L

  /**
   * A batch of messages will be emitted to PubSub when the batch reaches 10 MB.
   * We use 10MB because it is the maximum batch size allowed by PubSub.
   */
  val DefaultPubsubMaxBatchBytes = 10000000L

And to make more parameters of enrich-kinesis configurable (e.g. maxConcurrency and bufferSize)