This adds a new stream config setting 'message_key_fields' that maps
from key field paths to event field paths.  The key object
can then be hoisted out of event values.

E.g.

  message_key_fields:
    key_field_a: sub.field.a

Will result in a key object like

  { key_field_a: <value of event.sub.field.a>}

Keys are serialized as JSON strings.

librdkafka's default partitioner will produce null keys to random partitions, and
non-null keys by consistently hashing the key to a specific partition.
This will allow us to produce messages with the same key to the same parttition.

Bug: T318846
Change-Id: Iae478d7bc43107fc290bd1fc2bcbde08f42fce1c