poerm_grpc: Use metadata of Status structure in headers

[maksimtor]

Currently in poem_grpc we have structure Status which contains metadata. And also we have GrpcServer that uses Status::to_headers() method to extend response headers. This method does not use its metadata, and I actually can't see where we use Status metadata at all.
When I want to send error response in form of Status, I would like to send additional headers and I can't do it now. Can we consider using metadata from Status to send additional headers in response? Or if we don't want it, can we create additional field like 'custom_headers' in Status in order to retrieve them for use in response headers? It seems to be a small feature and I can make PR if idea is okay.

[sunli829]

Welcome PR, I would like to add a custom_headers method to Status.