Create custom image for tls cassandra

[rukai]

Cuts off ~2mins from cassandra_int_tests total runtime. (cuts ~1 min off of 2 integration tests)
also cuts 2min off the total runtime for CI benchmarks (1 TLS bench and every bench is run twice in CI)

Needed to create a separate image as we need to modify the config file to set TLS.

I also noticed the regular image cassandra.yaml had some experimental features enabled so I disabled those.

[conorbros]

I thought we just needed to provide the keystore to enable TLS?

[rukai]

We also need to set client_encryption_options to enabled

[github-actions]

0 benchmark regressed. 1 benchmark improved. Please check the benchmark workflow logs for full details: https://github.com/shotover/shotover-proxy/actions/runs/3238829362

  1 (1.00%) high mild
cassandra/tls_select    time:   [455.19 us 461.71 us 469.14 us]
                        thrpt:  [2.1316 Kelem/s 2.1659 Kelem/s 2.1969 Kelem/s]
                 change:
                        time:   [-35.075% -33.423% -31.877%] (p = 0.00 < 0.05)
                        thrpt:  [+46.793% +50.201% +54.023%]
                        Performance has improved.