Add Windows conformance runner and testee support (#28130)

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.
```

Closes #28130

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/28130 from dcodeIO:windows 4872b82d608dff91672bad47229bb60542a8971a
PiperOrigin-RevId: 968704895