This PR implements the status/result logger functions necessary interface with a Kafka REST Proxy service.
Specifically, this is compatible with the [Confluent KAFKA Rest Proxy Service ](https://docs.confluent.io/1.0/kafka-rest/docs/intro.html).
Add a config setting to allow copying message fields and decorations into Google Pub/Sub attributes, making it possible to use these values for subscription filters.
Adds Google Cloud PubSub logging for status and results.
This also changes the Write interface for logging modules to add a context.Context (only used by pubsub currently).
- Refactor configuration for logging to use separate plugins
- Move existing filesystem logging to filesystem plugin
- Create new AWS firehose plugin
- Update documentation around logging