Sending device model over ffi for client telemetry

[MaxHeimbrock]

This is probably a breaking change.

Send device_model if set from ffi, otherwise send "Unknown"

Alternatively we could add it to some room options proto, but this follows the same pattern as sdk and sdk version, which is also client telemetry and set in the LiveKitInitialize of the ffi.

Enables: livekit/client-sdk-unity#201

[ladvoc]

From sync: this is technically a breaking change since it changes the C ABI. However, given that other client info is passed this way, I would say we should do the same here for consistency.

[ladvoc]

Looking good so far, I've left a few questions and comments.

[ladvoc]

suggestion: Unlike SDK version, not all clients will be able to populate this field. We can store this as Option<String> on the Rust side, mapping a null pointer to Option::None.

[MaxHeimbrock]

How do I do the mapping here?

I tried:

device_model: {
            if device_model.is_null() {
                None
            } else {
                let s = CStr::from_ptr(device_model).to_string_lossy().into_owned();
                if s.is_empty() {
                    None
                } else {
                    Some(s)
                }
            }

And that did not work when not filling the field from Unity side. I again end up with garbage in logs:

[xianshijing-lk]

two nits, lgtm if you address them.

[MaxHeimbrock]

I moved the field over to the general signaling options instead of the SDK options.