Add Windows conformance runner and testee support

[dcodeIO]

Adds Windows support for the conformance runner and C++ conformance testee. For the conformance runner, this splits ForkPipeRunner into POSIX and Win32 implementations. The testee is switched to portable FILE* I/O, with binary mode on Windows.

Also makes two related changes:

• waitpid(..., WEXITED) is changed to waitpid(..., 0) because WEXITED is a waitid() flag. waitpid(..., 0) already waits for the child to terminate.
• Adds protobuf_configure_target to the CMake conformance targets so they get the necessary target flags, e.g. /utf-8, which is important on Windows.

Otherwise wires the change into CI so the Windows target functionally mirrors the existing macOS target.

Resolves this existing Windows TODO in fork_pipe_runner.cc:

// TODO: make this work on Windows, instead of using these
// UNIX-specific APIs.
//
// There is a platform-agnostic API in
//    src/google/protobuf/compiler/subprocess.h
//
// However that API only supports sending a single message to the subprocess.
// We really want to be able to send messages and receive responses one at a
// time:
//
// 1. Spawning a new process for each test would take way too long for thousands
//    of tests and subprocesses like java that can take 100ms or more to start
//    up.
//
// 2. Sending all the tests in one big message and receiving all results in one
//    big message would take away our visibility about which test(s) caused a
//    crash or other fatal error.  It would also give us only a single failure
//    instead of all of them.