Add example to demonstrate how to create a CA and a certificate signed with that CA.

Basically I want to be able to generate the ca.crt/ca.key and redis.crt/redis.key files that this script generates https://github.com/redis/redis/blob/unstable/utils/gen-test-certs.sh#L5+L6
But I cant figure out how to from looking at the API, it seems like it should be possible though?