add basic sasl_ssl support:

[naude-r]

• add security_protocol to authConfig. a single value, SASL_SSL, is supported for now. this allows one to enable TLS support
• add sasl_mechanism to authConfig. currently we check for PLAIN and SCRAM-SHA256/SCRAM-SHA512
• setupAuthTLS can now handle both client certs and the old "1 way" setup

[jvansanten]

Very nice! I just ran across a case where I needed this, and implemented it myself without checking for open PRs first. Shame on me.

Some improvements I might suggest:

• Add a block to README.md describing the required fields and their permitted values for this auth mode
• Use the same spelling as sarama for the auth mechanisms, e.g. SCRAM-SHA-256 instead of SCRAM-SHA256. This could reduce friction for users following examples written for librdkafka.

@fgeller This would be very nice to have merged.