Support m.call.reject

Start the migration to v1 VoIP by supporting m.call.reject, which
we'll send if the caller says they're v1. Our version stays as v0
for now, until we speak the rest of v1.

Honour the default reaosn in a hangup being user_hangup.

src/@types/event.ts

@@ -45,6 +45,7 @@ export enum EventType {
     CallCandidates = "m.call.candidates",
     CallAnswer = "m.call.answer",
     CallHangup = "m.call.hangup",
+    CallReject = "m.call.reject",
     KeyVerificationRequest = "m.key.verification.request",
     KeyVerificationStart = "m.key.verification.start",
     KeyVerificationCancel = "m.key.verification.cancel",