+per #17579 #17617 Introduces EventAdapter

+ per plugin scoped adapters
+ could be swapped during runtime
+per EventAdapter now has manifest and is configurable ai la serializers
+ json examples in docs
+ including "completely manual" example in case one wants to add
  metadata TO the persisted event
+ better error reporting when misconfigured bindings
+ manifest is handled by in memory plugin
- did not check if it works with LevelDB plugin yet
> TODO: json example uses Gson, as that's simplest to do, can we use
+per allows 1:n adapters, multiple adapters can be bound to 1 class

akka-stream/src/test/scala/akka/persistence/stream/PersistentPublisherSpec.scala

@@ -24,7 +24,7 @@ object PersistentPublisherSpec {
   }
 }
 
-class PersistentPublisherSpec extends AkkaSpec(PersistenceSpec.config("leveldb", "ViewProducerSpec", serialization = "off")) with PersistenceSpec {
+class PersistentPublisherSpec extends PersistenceSpec(PersistenceSpec.config("leveldb", "ViewProducerSpec", serialization = "off")) {
   import PersistentPublisherSpec._
 
   val numMessages = 10