Windows: add ucrt support (imports + _getpid)

[zamderax]

Adds canImport(ucrt) branches in RetryDelaySequence and Syscalls;

I added uses of ucrt._getpid on Windows and I verified that swift build and swift build --build-tests succeed on Windows with Swift 6.2

Some Context:

This allows me and others to create a custom gRPC ClientTransport that isn't based off of swift-nio. It looks like adding windows support to Swift NIO is a very heavy lift, so in the interim it's possible for me to create a ClientTransport that uses https://learn.microsoft.com/en-us/windows/win32/winhttp/about-winhttp and use .when and #if os(windows) in my code to swap out the transport so that I can have a Swift Client that uses the WinHTTP backend.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: glbrntt / name: George Barnett (0bf5bb6)

[glbrntt]

Thank you!

Thanks for providing the context too, sounds like a fun project