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/test.rs

@@ -7,3 +7,5 @@ pub(crate) use dispatcher::*;
 pub(crate) use display::*;
 pub(crate) use platform::*;
 pub(crate) use window::*;
+
+pub use platform::TestScreenCaptureSource;