What does this PR do?

This commit adds support for topic selectors to provide parity with the existing beats Kafka output - see https://www.elastic.co/guide/en/beats/filebeat/current/kafka-output.html#topic-option-kafka and https://www.elastic.co/guide/en/beats/filebeat/current/kafka-output.html#topics-option-kafka for details on how to use this function.

This PR continues to use the outil.Selector from libbeat as used in the beats Kafka output

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
  - [ ] I have made corresponding changes to the documentation
  - [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
  - [ ] I have added an entry in CHANGELOG.md or CHANGELOG-developer.md.

How to test this PR locally

Testing can be done the same way as #147, with changes for the kafka output:

Run a local Kafka running on port 9092, and using metricbeat with:

output.shipper:
  server: "localhost:50052"

And the following settings in elastic-agent-shipper.yml

output.kafka:
  enabled: true
  hosts: ["localhost:9092"]
  topics:
  - topic: "network"
    when.contains:
      metricset.name: "network"
  - topic: "process"
    when.contains:
      metricset.name: "process"
  - topic: "error"
    default:  

And using kcat to pull messages out of the various topics to prove the correctness of delivery.

kcat -b localhost:9092 -G mygroup <TOPIC_NAME>

Related issues

Closes #167