Predefine agent policies in the Kibana config

[mtojek]

Kibana Fleet team is going to deprecate and remove default agent policies in elastic/kibana#108456.

It means that the elastic-package stack up won't boot up eventually if the 8.0.0-SNAPSHOT 8.1.0-SNAPSHOT image is selected. The goal of this issue is to predefine policies in the kibana.config.yml (or a file close to it) and use them instead ones created by Elastic Agents.

@juliaElastic provided us also sample policies: here (thanks!)

[juliaElastic]

@joshdover
While testing the predefined policy changes in elastic-package, we discovered that elastic-agent has a fallback to is_default policy flag in case enrollment token is not specified: https://github.com/elastic/beats/blob/master/x-pack/elastic-agent/pkg/agent/cmd/container.go#L544

Elastic-agent uses the same docker-compose-stack.yml for all versions, so we have to use the default flags in kibana 8 config (setting policy id envvar would break pre-8 versions).

Any concerns with this? My only concern is that if we plan to remove the is_default flags completely, we would have to come back to elastic-agent again and find a solution that works with older versions too.

The alternative would be to either use different versions of docker-compose file, or use the preconfigured policies in pre-8 versions as well.

[mtojek]

It looks like we're safe to merge the PR. The flag is_default worked well.