=per #16009 Serialize SnapshotHeader without Java serialization (for validation)

* it is backwards compatible, i.e. it can read old snapshots

(cherry picked from commit 690905eac9619da91d7b1fcbc633d1fa2e411ee7)

Conflicts:
	akka-persistence/src/test/scala/akka/persistence/serialization/SerializerSpec.scala