fix service_provider_sid undefined (#101)

Co-authored-by: p.souza <p.souza@cognigy.com>

lib/call-session.js

@@ -99,6 +99,10 @@ class CallSession extends Emitter {
     this._mediaReleased = false;
   }
 
+  get service_provider_sid() {
+    return this.req.locals.service_provider_sid;
+  }
+
   get account_sid() {
     return this.req.locals.account_sid;
   }