Implement re-negotiation

[SystemKeeper]

Corresponding frontend commit: nextcloud/spreed@48a74da

We need to make sure that we send the sid in this request, when we want to re-negotiate:

talk-ios/NextcloudTalk/NCExternalSignalingController.m

Lines 498 to 515 in 18cd47e

	- (void)requestOfferForSessionId:(NSString *)sessionId andRoomType:(NSString *)roomType
	{
	NSDictionary *messageDict = @{
	@"type": @"message",
	@"message": @{
	@"recipient": @{
	@"type": @"session",
	@"sessionid": sessionId
	},
	@"data": @{
	@"type": @"requestoffer",
	@"roomType": roomType
	}
	}
	};
	[self sendMessage:messageDict withCompletionBlock:nil];
	}