tests(plugins/rate-limiting) test ssl connection

[ADD-SP]

Summary

This is a retry for #8617, which was rolled back by #8649 because it was unstable.

The SSL connection between the rate-limiting plugin and Redis was not tested.

Difference with #8617

• Fix a tiny bug in the rate-limiting plugin, if the config.redis_database is 0, the default connection pool should be used.
• Reissue the server.crt to avoid the server.crt expiration exceed the ca.crt.

Full changelog

• Fix a tiny bug in the rate-limiting plugin, if the config.redis_database is 0, the default connection pool should be used.
• Add relevant testing strategies to spec/03-plugins/23-rate-limiting/05-integration_spec.lua.
• Update .github/workflows/build_and_test.yml to enable SSL for Redis in CI
• Create folder spec/fixtures/redis and add the following files
  • ca.key: Private key for the root certificate
  • ca.crt: Root Certificate
  • server.key: Private key for Redis server certificate
  • server.crt: Redis server certificate
  • docker-entrypoint.sh: To override the default

These certificates should be added to Kong-Pongo and gojira to make it easy for developers to run tests locally.

Related PR: Kong/gojira#45, Kong/kong-pongo#270