Return the call in the response for the start function

[Nikhil-Kulkarni]

Background We are unable to associate a call with a userId with the iOS SDK because vapi.start does not return the id of the call. It is currently a void function. The typescript SDK returns a Promise<Call>. I'm updating the iOS SDK to return the WebCallResponse and adding the id returned by /call/web to the WebCallResponse. This currently does not have parity with the typescript SDK (which returns the entire Call) but I'll leave that to you guys

Changes

• Add id to WebCallResponse model
• Return async WebCallResponse (equivalent to Promise<Call> in the typescript SDK) from vapi.start(...)