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>

.github/workflows/ci.yml

@@ -129,6 +129,7 @@ jobs:
         run: |
           cargo build --workspace --bins --all-features
           cargo check -p gpui --features "macos-blade"
+          cargo check -p workspace --features "livekit-cross-platform"
           cargo build -p remote_server
 
   linux_tests: