Add SASL/SCAM support to Kafka transport.

[Packetslave]

This pull request adds the optional -transport.kafka.scram flag, which tells the Kafka transport to use SASL/SCRAM for authentication. This allows goflow2 to work with AWS Managed Service for Kafka (MSK) which does not support SASL/PLAIN.

Tested against AWS MSK:

./goflow2 \
    -transport=kafka \
    -transport.kafka.topic=ExampleTopic \
    -format=pb \
    -listen netflow://:2155 \
    -transport.kafka.brokers=$BROKERS \
    -transport.kafka.tls \
    -transport.kafka.sasl \
    -transport.kafka.scram=sha512
INFO[0000] Authenticating as user 'goflow'...
INFO[0000] Starting GoFlow2
INFO[0000] Starting collection                           hostname= port=2155 scheme=netflow

[lspgn]

Thanks a lot for fixing this!
Apologies for the delay in reply.

My suggestion, while I know it's a breaking change, would be to change transport.kafka.sasl to a string. It could be set to plain, scram-256 or scram-512.
Would you be able to change this? Rest looks good.