feat: support message key for Kafka external stream

[zliang-min]

PR checklist:

• Did you run ClangFormat ?
• Did you separate headers to a different section in existing community code base ?
• Did you surround proton: starts/ends for new code in existing community code base ?

Please write user-readable short description of the changes:

closes #433

Added a new external stream setting message_key which is an expression that returns a string value, the values return by the expression will be used as the message key for each row. Examples:

-- use a column
CREATE EXTERNAL STREAM example_one (
  one string,
  two int32
) SETTINGS type='kafka',...,message_key='one'

-- use a complex expression
CREATE EXTERNAL STREAM example_two (
  one string,
  two int32
) SETTINGS type='kafka',...,message_key='split_by_string(\',\', one)[1]'

message_key can be used with sharding_expr together, and sharding_expr will take priority (exactly what a Kafka client does).

Together with this change, a couple more changes are worth mentioning:

• the random sharding logic has been simplified, it now relies on librdkafka for partitioning.
• it now uses rd_kafka_produce_batch to send messages to Kafka, which is expected to be more efficient ( at least, it removes the overhead of calling rd_kafka_produce for each message ). This should benefit one_message_per_row even more.

[chenziliang]

do we need all of this cast ?

[chenziliang]

Same for payload

[zliang-min]

The difference here is that key.data has type of const char *, if we remove the const_cast, it will report Cannot initialize a member subobject of type 'void *' with an lvalue of type 'const char *'. But the static_cast` can be removed.