Fix: Loader cannot start with InitialPosition = AT_TIMESTAMP

[istreeter]

The S3 loader seems to allow loading from a given point in time in the kinesis stream. The configuration to get this working is:

  "position": {
    "at_timestamp": {
      "timestamp": "2022-06-06T06:06:06Z"
    }
  }

But we get an exception like:

	2022-08-03T15:22:36.009+01:00	Exception in thread "main" java.lang.IllegalArgumentException: Invalid InitialPosition: AT_TIMESTAMP
	2022-08-03T15:22:36.009+01:00	at com.amazonaws.services.kinesis.clientlibrary.lib.worker.InitialPositionInStreamExtended.newInitialPosition(InitialPositionInStreamExtended.java:68)
	2022-08-03T15:22:36.009+01:00	at com.amazonaws.services.kinesis.clientlibrary.lib.worker.KinesisClientLibConfiguration.<init>(KinesisClientLibConfiguration.java:708)
	2022-08-03T15:22:36.009+01:00	at com.snowplowanalytics.s3.loader.connector.KinesisSourceExecutor.getKCLConfig(KinesisSourceExecutor.scala:93)
	2022-08-03T15:22:36.009+01:00	at com.snowplowanalytics.s3.loader.connector.KinesisSourceExecutor.initialize(KinesisSourceExecutor.scala:121)
	2022-08-03T15:22:36.009+01:00	at com.snowplowanalytics.s3.loader.connector.KinesisSourceExecutor.<init>(KinesisSourceExecutor.scala:57)
	2022-08-03T15:22:36.009+01:00	at com.snowplowanalytics.s3.loader.S3Loader.run(S3Loader.scala:59)
	2022-08-03T15:22:36.009+01:00	at com.snowplowanalytics.s3.loader.S3Loader.run$(S3Loader.scala:41)
	2022-08-03T15:22:36.009+01:00	at com.snowplowanalytics.s3.loader.S3Loader$.run(S3Loader.scala:127)
	2022-08-03T15:22:36.009+01:00	at com.snowplowanalytics.s3.loader.Main$.$anonfun$main$1(Main.scala:51)
	2022-08-03T15:22:36.009+01:00	at com.snowplowanalytics.s3.loader.Main$.$anonfun$main$1$adapted(Main.scala:51)
	2022-08-03T15:22:36.009+01:00	at com.snowplowanalytics.s3.loader.MainPlatform.withConfig(Main.scala:38)
	2022-08-03T15:22:36.009+01:00	at com.snowplowanalytics.s3.loader.MainPlatform.withConfig$(Main.scala:33)
	2022-08-03T15:22:36.009+01:00	at com.snowplowanalytics.s3.loader.Main$.withConfig(Main.scala:49)
	2022-08-03T15:22:36.009+01:00	at com.snowplowanalytics.s3.loader.Main$.main(Main.scala:51)
	2022-08-03T15:22:36.009+01:00	at com.snowplowanalytics.s3.loader.Main.main(Main.scala)

The exception happens because of how we pass the initial position into the constructor... but the constructor (not controlled by us) only allows latest or trim_horizon because of this line here.

The solution is to always pass InitialPositionInStream.LATEST into the constructor, which is OK because we always amend the initial position immediately after calling the constructor.