Add getter methods for mid and paused in TrackLocalContext

[h3poteto]

Dear maintainers,
Thank you for your efforts in maintaining this project.

I have added two getter methods to retrieve the mid and paused fields from the TrackLocalContext struct.

In my project, I am implementing a custom TrackLocal similar to TrackLocalStaticRTP. The bind method provides a TrackLocalContext, but currently, I am unable to access all its fields because they are marked as pub(crate).

webrtc/webrtc/src/track/track_local/track_local_static_rtp.rs

Line 177 in de46a53

async fn bind(&self, t: &TrackLocalContext) -> Result<RTCRtpCodecParameters> {

To address this, I have introduced getter methods for the mid and paused fields, ensuring that these values can be accessed outside the crate while maintaining encapsulation.

I hope this change aligns with the project's design principles and proves helpful for others with similar use cases. Please let me know if there are any concerns or adjustments needed.

Thank you for considering this contribution.