Expose the event ID of a call membership (#4395)

This is in line with the other information we're already exposing, such as the event's sender and timestamp. We want this in order to play around with adding reactions to the membership event.

src/matrixrtc/CallMembership.ts

@@ -128,6 +128,10 @@ export class CallMembership {
         return this.parentEvent.getSender();
     }
 
+    public get eventId(): string | undefined {
+        return this.parentEvent.getId();
+    }
+
     public get callId(): string {
         return this.membershipData.call_id;
     }