Use LiveKit's Rust SDK on Linux while continue using Swift SDK on Mac (#21550)

Similar to #20826 but keeps the Swift implementation. There were quite a
few changes in the `call` crate, and so that code now has two variants.

Closes #13714

Release Notes:

- Added preliminary Linux support for voice chat and viewing
screenshares.

---------

Co-authored-by: Kirill Bulatov <mail4score@gmail.com>
Co-authored-by: Kirill Bulatov <kirill@zed.dev>
Co-authored-by: Mikayla <mikayla@zed.dev>

crates/gpui/src/platform/linux.rs

@@ -20,3 +20,5 @@ pub(crate) use text_system::*;
 pub(crate) use wayland::*;
 #[cfg(feature = "x11")]
 pub(crate) use x11::*;
+
+pub(crate) type PlatformScreenCaptureFrame = ();