Support specifying a path to a CA certificate to trust

[nogweii]

Describe the problem
Right now, there is no way to specify (that I can see) the path to a PEM encoded certificate that contains the redis/valkey's root CA.

Running the exporter
I'm not running it right now, due to other issues in the valkey-helm chart I've run into. But applicable to this, it would be setting the environment variables $REDIS_ADDR and $REDIS_EXPORTER_SKIP_TLS_VERIFICATION.

Expected behavior
There is an environment variable, akin to $REDIS_EXPORTER_TLS_SERVER_CA_CERT_FILE that tells redis_exporter to trust that CA.

Additional context
Running in kubernetes, in my homelab, via the valkey-helm chart.