Generate redis TLS certs at test time

[rukai]

This makes progress towards #538

Uses https://github.com/est31/rcgen to generate the certs without a dependency on openssl
Although we could get away with using openssl I would like to move to rustls one day so better to use pure rust solutions where possible.

The generated certs end up a little different to those originally generated by https://github.com/redis/redis/blob/unstable/utils/gen-test-certs.sh
Here we have the difference between the old and new redis.crt

I tried to make it use the same RSA 256 algorithm previously used but rcgen does not support that yet due to: briansmith/ring#219

But the modified tests continue to pass so I dont see any problem with using a different algorithm.

To be done in follow up PRs:

• rename redis-tls/tls_keys -> redis-tls/certs to match the naming scheme used by cassandra
• generate cassandra certs at test time

But they are low priority, I was only working on this last week due to being blocked on other work.

[conorbros]

Just that one comment, LGTM.