try to resolve `FIXME share trailers through matval`

[laglangyue]

source code https://github.com/apache/pekko-grpc/blob/83bdd04b1eb81c84153debab4a0945a6d52cc345/runtime/src/main/scala/org/apache/pekko/grpc/internal/PekkoNettyGrpcClientGraphStage.scala#L169-L176

I've submit a pr

[laglangyue]

I went to learn about GRPC, header=>message=>trailers We stored the header and future [trailers] through GrpcResponseMetadata We have completed the promise [GrpcResponseMetada] in onHeader, so we are unable to assign a value to the trainers, So the original author defined another Promise [trailers] When on Hedaer, GrpcResponseMetadata is in a semi completed state OnClose is the final state

[laglangyue]

I think this FIXME may not be necessary, but there is a lot of repetitive code about GrpcResponseMetada, which can be constructed using a builder pattern to create GrpcResponseMetada, with Header, with Message, with Close (with Trailers)

I have tried writing some code locally, but I still need some deeper exploration

class GrpcResponseMetadataImpl(
    grpcHeaders: io.grpc.Metadata, trailersPromise: Promise[io.grpc.Metadata]) extends GrpcResponseMetadata {

  override def headers: pekko.grpc.scaladsl.Metadata =
    MetadataImpl.scalaMetadataFromGoogleGrpcMetadata(grpcHeaders)
  override def getHeaders(): pekko.grpc.javadsl.Metadata =
    MetadataImpl.javaMetadataFromGoogleGrpcMetadata(grpcHeaders)
  override def trailers: Future[pekko.grpc.scaladsl.Metadata] =
    trailersPromise.future.map(MetadataImpl.scalaMetadataFromGoogleGrpcMetadata)(ExecutionContexts.parasitic)
  override def getTrailers(): CompletionStage[pekko.grpc.javadsl.Metadata] =
    trailersPromise.future.map(MetadataImpl.javaMetadataFromGoogleGrpcMetadata)(ExecutionContexts.parasitic)
      .asJava
}