Add proxy to cluster TLS support (--cluster-ca-file)

[mpenick]

Adds basic support for proxy to cluster TLS. Use --cluster-ca-file to
enable. To use a client certificate use --cluster-cert-file and
--cluster-key-file. This is useful for Cassandra clusters that use
TLS.

[kamek-pf]

Hey there, thanks for looking into this !
Running this command:

./cql-proxy \
    --contact-points=subdomain.amazonaws.com \
    --username "something" \
    --password "something" \
    --port 9142 \
    --cluster-ca-file="/path/to/sf-class2-root.crt"

still gives me the following error:
cql-proxy: error: unable to connect to cluster context deadline exceeded