Added optional config for Kibana's Elastic Agent / Fleet Server

The ECK Operator can deploy Elastic Agent if Kibana is set up with certain config.xpack.fleet.* keys. This PR adds those keys as described here: https://www.elastic.co/guide/en/cloud-on-k8s/2.7/k8s-elastic-agent-fleet-quickstart.html

Users who want to deploy Elastic Agent can just uncomment & edit this new kibana.agents section in values.yaml and then apply their Agent manifests separately from Big Bang.

Without this PR, Big Bang users who wish to deploy Elastic Agent must either patch the Big Bang Kibana object to add the necessary fields, or deploy a 2nd Kibana instance with Agent support.

This should not be a breaking change for existing users of the elasticsearch-kibana project.