Docs: add moduledocs for Ockam.Stream.Client.Consumer and Ockam.Stream.Client.Publisher

[hairyhum]

Stream client components could use some documentation

Stream protocol documentation: https://github.com/ockam-network/proposals/tree/main/design/0009-stream-protocol Example stream client usage: https://github.com/ockam-network/ockam/blob/develop/implementations/elixir/ockam/ockam/lib/ockam/examples/stream/stream.ex