Add metadata field to Kafka source

[mwylde]

Adds key as a metadata field to the kafka source, allowing the message key to be injected into queries:

create table kafka (
    key BYTEA GENERATED ALWAYS AS (metadata('key')) STORED,
    topic TEXT GENERATED ALWAYS AS (metadata('topic')) STORED,
    value TEXT
)  with (
    connector = 'kafka',
    bootstrap_servers = 'localhost:9092',
    topic = 'logs',
    type = 'source',
    format = 'raw_string'
);

select cast(key as TEXT), value, topic from kafka;