Implement WriteEvent on file output so that it can be used by 'inputs' components

[netixx]

WriteEvent is used by the "inputs" components to output their data.

Without this implementation,
things like

inputs:
  nats-replay:
    type: nats
    address: nats-server:4222
    subject: replay
    debug: true
    buffer-size: 1
    outputs:
      - file-dump

outputs:
  file-dump:
    type: file
    format: event
    debug: true
    split-events: true
    filename: /output.json
    concurrency-limit: 1

wont actually output anything to the file.

[netixx]

Thanks!