Split out generic portal / pipewire code

It will be reused by the video capture portal / pipewire backend.

Bug: webrtc:13177
Change-Id: Ia1a77f1c6e289149cd8a1d54b550754bf192e62e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/263721
Reviewed-by: Mark Foltz <mfoltz@chromium.org>
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Salman Malik <salmanmalik@google.com>
Cr-Commit-Position: refs/heads/main@{#38487}

modules/portal/pipewire_stub_header.fragment

@@ -0,0 +1,8 @@
+// The extra include header needed in the generated stub file for defining
+// various PipeWire types.
+
+extern "C" {
+
+#include <pipewire/pipewire.h>
+
+}