Add clientCert to APIGatewayProxyRequestEvent

[deki]

As per https://aws.amazon.com/blogs/compute/introducing-mutual-tls-authentication-for-amazon-api-gateway/ and https://docs.aws.amazon.com/apigateway/latest/developerguide/set-up-lambda-proxy-integrations.html clientCert needs to be added to https://github.com/aws/aws-lambda-java-libs/blob/master/aws-lambda-java-events/src/main/java/com/amazonaws/services/lambda/runtime/events/APIGatewayProxyRequestEvent.java#L431

[tulsidas]

Hi, can this be included in a 3.x release? I need this feature specifically but it's not present in the latest release (3.11.0)
Thanks!