Support Kafka message transformation

[jfallows]

Describe the desired outcome from the user's perspective
As an application developer, I need to transform messages received from a Kafka topic to present via mqtt-kafka, http-kafka or sse-kafka mappings.

Acceptance criteria

• transforms should not be required
• transforms should be aligned with Kafka Connect SMTs
• transforms should be evaluated in configured order

Additional context
Suggested configuration for transforms.

  kafka_cache_server:
    type: kafka
    kind: cache_server
    options:
        topics:
          - name: my-transformed-topic
            transforms:
              - extract-key: ${message.key.id}
              - extract-headers:
                'zilla:identity': ${message.value.user_id}
...

See also Kafka Connect transformations.

Tasks

• Support extract-headers kafka message transform #1175
• Support extract-key kafka message transform #1176
• ...