support reading akka-persistence snapshots

[pjfanning]

• aim is to get this working for 1.0.x and forward fit to main branch
• see make entries with legacy class names backwards compatible scullxbones/pekko-persistence-mongo#14 (comment)

@SakulK would you be able to share a persisted akka snapshot? In base64 format, maybe. It would be useful to have some sort of test.

[SakulK]

Here, along with the code to generate it:

package akka.persistence

import akka.actor.ActorSystem
import akka.serialization.{ SerializationExtension, SerializerWithStringManifest }
import akka.persistence.serialization.Snapshot
import akka.persistence.fsm.PersistentFSM.PersistentFSMSnapshot
import java.util.Base64

object MakeTestSnapshot extends App {
  val system = ActorSystem()
  val serialization = SerializationExtension(system)
  val bytes = serialization.serialize(Snapshot(PersistentFSMSnapshot[String]("test-identifier", "test-data", None)))
  val base64 = Base64.getEncoder.encode(bytes.get)
  println(new String(base64)) // PAAAAAcAAABha2thLnBlcnNpc3RlbmNlLmZzbS5QZXJzaXN0ZW50RlNNJFBlcnNpc3RlbnRGU01TbmFwc2hvdAoPdGVzdC1pZGVudGlmaWVyEg0IFBIJdGVzdC1kYXRh
}

and test deserialization on pekko:

import org.apache.pekko.actor.ActorSystem
import org.apache.pekko.serialization.{ SerializationExtension, SerializerWithStringManifest }
import java.util.Base64
import org.apache.pekko.persistence.serialization.Snapshot

val system = ActorSystem()
val serialization = SerializationExtension(system)
val bytes = Base64.getDecoder.decode("PAAAAAcAAABha2thLnBlcnNpc3RlbmNlLmZzbS5QZXJzaXN0ZW50RlNNJFBlcnNpc3RlbnRGU01TbmFwc2hvdAoPdGVzdC1pZGVudGlmaWVyEg0IFBIJdGVzdC1kYXRh")
val result = serialization.deserialize(bytes, classOf[Snapshot])

[nvollmar]

lgtm