Support m.call.select_answer

Only send one if the answer has a party_id set

Also change some event types to the enum and rename receivedAnswer
to be more consistent

src/@types/event.ts

@@ -46,6 +46,7 @@ export enum EventType {
     CallAnswer = "m.call.answer",
     CallHangup = "m.call.hangup",
     CallReject = "m.call.reject",
+    CallSelectAnswer = "m.call.select_answer",
     KeyVerificationRequest = "m.key.verification.request",
     KeyVerificationStart = "m.key.verification.start",
     KeyVerificationCancel = "m.key.verification.cancel",