This updates pw_transfer's integration tests to use local files as
transfer targets. This makes it simpler to coordinate between C++ and
Python and verify the results of transfer operations.
As part of this, a stream reader and writer that operate on files using
the C++ standard library are added to pw_stream.
Change-Id: I93efe3526ea9569b2f67a371fc89872cca0492a8
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/57342
Commit-Queue: Alexei Frolov <frolv@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
