Windows: add ucrt support (imports + _getpid) (#29)

Windows support: import ucrt in RetryDelaySequence and Syscalls; use _getpid on Windows; build and tests succeed

Author: zamderax

Sources/GRPCCore/Call/Client/Internal/RetryDelaySequence.swift

@@ -21,6 +21,8 @@ public import Android  // should be @usableFromInline
 public import Glibc  // should be @usableFromInline
 #elseif canImport(Musl)
 public import Musl  // should be @usableFromInline
+#elseif canImport(ucrt)
+public import ucrt  // should be @usableFromInline
 #else
 #error("Unsupported OS")
 #endif

Sources/GRPCInProcessTransport/Syscalls.swift

@@ -22,6 +22,8 @@ private import Android  // should be @usableFromInline
 private import Glibc  // should be @usableFromInline
 #elseif canImport(Musl)
 private import Musl  // should be @usableFromInline
+#elseif canImport(ucrt)
+private import ucrt  // should be @usableFromInline
 #else
 #error("Unsupported OS")
 #endif
@@ -40,6 +42,11 @@ enum System {
     #elseif canImport(Musl)
     let pid = Musl.getpid()
     return Int(pid)
+    #elseif canImport(ucrt)
+    let pid = ucrt._getpid()
+    return Int(pid)
+    #else
+    return 0
     #endif
   }
 }