support inworld tts (#1262)

* support inworld tts

* wip

* wip

lib/session/call-session.js

@@ -1109,6 +1109,13 @@ class CallSession extends Emitter {
             options: credential.options
           };
         }
+        else if ('inworld' === vendor) {
+          return {
+            api_key: credential.api_key,
+            model_id: credential.model_id,
+            options: credential.options
+          };
+        }
         else if ('assemblyai' === vendor) {
           return {
             speech_credential_sid: credential.speech_credential_sid,