Support Avro Doc Comments materialize_sink_kafka

[dehume]

Fixes #359

Add two new attributes to the materialize_sink_kafka, avro_doc_type and avro_doc_column to support Avro comments. Adds the following syntax to the SQL

CREATE SINK avro_sink
  ...
  (
    DOC ON TYPE t = 'top-level comment for avro record in both key and value schemas',
    KEY DOC ON COLUMN t.c1 = 'comment on column only in key schema',
    VALUE DOC ON COLUMN t.c1 = 'comment on column only in value schema'
  )

[benesch]

This is still WIP, so let me know if this is too deep too soon, but I think we should track the SQL syntax more closely here. I think you want something more like:

format {
    avro {
        doc_on_type {
            database_name = "..."
            schema_name = "..."
            type_name = "..."
            doc = "..."
            # At least one of key or value must be true.
            key = {true | false}
            value = {true | false}
        }

        doc_on_column {
            database_name = "..."
            schema_name = "..."
            item_name = "..."
            column_name = "..."
            doc = "..."
            # At least one of key or value must be true.
            key = {true | false}
            value = {true | false}
        }
    }
}

[dehume]

Based on docs, unsure if we need to also support aliases for the object reference. Or if it should just always make the connection.

[dehume]

Open to better descriptions for all these.

[dehume]

The acceptance test failing is within Kafka Sources, not sinks. Must be due to some of the shared structs within format_specs