Implement sinks

[purplefox]

Sinks will allow the output from materialized views or sources to be sent to a Kafka topic.

Similarly to sources, they will be declared using a CREATE SINK command. Something like:

create sink my_sink_name from source_or_mv_name
with (
    brokername = "testbroker",
    topicname = "topic_to_send_the_data_to",
    headerencoding = "json",
    keyencoding = "json",
    valueencoding = "json",
    columnselectors = (
        "k.k0",
        "v.v1",
        "v.v2",
        "v.v3",
        "v.v4",
        "v.v5"
    )
);