Update the sdp field in the RTCIceCandidate class to candidate

[970263611]

During actual development, we discovered that an incorrect field description caused the backend to return an incorrect ice-candidate to the frontend. This prevented the frontend from correctly initiating a connection request to the backend, resulting in a failed WebRTC connection establishment.

We tested this using the Go language's Pion component and found that it connected successfully. Comparing the message packets revealed the issue was with the field name.

Below is a packet capture. You can see that using the Pion component, the frontend quickly initiates a connection request to the backend, while the WebRTC-Java component does not initiate any requests. After manually changing the sdp name to candidate on the frontend, the request could be initiated correctly.