Extensible SASL support

[awr]

There is a need for SASL support at the Connection level (to be triggered right after API version support is triggered), with a default implementation of PLAIN. This should be extensible so any user of the client can provide their own implementation if it isn't included in the library.

[awr]

See http://kafka.apache.org/protocol.html#sasl_handshake for the handshake to use

[awr]

Also see https://cwiki.apache.org/confluence/display/KAFKA/KIP-43%3A+Kafka+SASL+enhancements for the SASL enhancements