Add headless Linux build path

[mikefaille]

Summary

• make headless Linux an explicit supported path without changing the default desktop build behavior
• re-expose glib-main-loop through livekit while keeping it enabled by default for desktop-oriented consumers
• add LK_HEADLESS=1 handling in webrtc-sys/build.rs to skip Linux desktop capturer sources, GLib/GIO probing, and X11 desktop-capture linkage
• pin direct desktop examples back to glib-main-loop explicitly so example behavior remains desktop-friendly

Why

On Linux, libwebrtc enables glib-main-loop by default and livekit inherited that behavior transitively. That made it impossible to build an intentionally headless Linux configuration without pulling in the GLib desktop path. This change preserves the existing default behavior while making the headless path explicit and supported.

Headless Linux requires both pieces:

• LK_HEADLESS=1 changes the Linux native build path in webrtc-sys
• --no-default-features -F tokio disables the default desktop-oriented glib-main-loop feature in the Rust dependency graph

In other words, LK_HEADLESS=1 by itself is not intended to change the default feature selection for livekit.

Validation

• cargo build -p livekit
• LK_HEADLESS=1 cargo build -p livekit --no-default-features -F tokio
• cargo tree -p livekit -i glib shows glib in the default build
• LK_HEADLESS=1 cargo tree -p livekit --no-default-features -F tokio -i glib shows glib absent from the graph

[CLAassistant]

All committers have signed the CLA.

[mikefaille]

There is one test failing but I don't part it's from this pr. @typester @ladvoc and @reenboog

[reenboog]

Looks good to me. That failing test has been there before, no worries. There's a merge conflict – please resolve it and we're good to go.