pw_stream: Add a ReaderWriter base class

Add a `class ReaderWriter` base class that combines interfaces of
pw::stream::Reader/Writer.

Change-Id: I20a8d506523483bb9c61f3990ffea2b79a1c1a3c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/48462
Commit-Queue: Yecheng Zhao <zyecheng@google.com>
Reviewed-by: David Rogers <davidrogers@google.com>
Reviewed-by: Ali Zhang <alizhang@google.com>