Allow setting headers from Serializer

Existing API (which I suppose was modeled after java API) have immutable Headers as an argument to serialize:

def serialize(topic: String, headers: Headers, a: A): F[Array[Byte]]

Immutability of Headers makes it impossible to set headers when deserializing a message. Probably, Headers should be moved to return position of serialize:

def serialize(topic: String, a: A): F[(Array[Byte], Headers)]

[filosganga]

I don’t believe the java serialiser allows to set headers. Does it?

[bplommer]

If the problem is just that the output of Headers#toJava is immutable, I think the following should work - it converts the headers to the Java class using a constructor that makes a (shallow) defensive copy.

Does this look like it will solve the issue? If so I’m happy to incorporate it into the library, but could you first try it out and let me know how it goes?

import scala.jdk.CollectionConverters._

def fromJava[F[_], A](serializer: KafkaSerializer[A])(
  implicit F: Sync[F]
): Serializer[F, A] =
  Serializer.instance[F, A] { (topic, headers, a) =>
    F.pure(
      serializer.serialize(
        topic,
        new RecordHeaders((headers.toChain: Chain[KafkaHeader]).toIterable.asJava),
        a
      )
    )
  }