CI Automation to keep in sync elastic-agent manifests of github and kibana repos

[gizas]

Describe the enhancement:

Currently Cloud Native team maintains only standalone and managed elastic agent manifests of github
See https://github.com/elastic/elastic-agent/tree/main/deploy/kubernetes

Kibana team maintains another version of same files in Kibana in order to provide the different Fleet functionalities:
Standalone Manifest: https://github.com/elastic/kibana/blob/main/x-pack/plugins/fleet/server/services/elastic_agent_manifest.ts#L8

Managed Manifest: https://github.com/elastic/kibana/blob/main/x-pack/plugins/fleet/server/services/elastic_agent_manifest.ts#L274

This task will try to automate the syncing of above manifests in https://beats-ci.elastic.co/ when a new PR opens in github under https://github.com/elastic/elastic-agent/tree/main/deploy/kubernetes

Describe a specific use case for the enhancement or feature:

1. Developer updates https://github.com/elastic/elastic-agent/tree/main/deploy/kubernetes/elastic-agent--kubernetes.yaml manifest
2. PR is merged
3. CI job is triggered nightly / (or manually) and will check the date of change of main branch
4. Will open a new PR to for the relevant https://github.com/elastic/kibana/blob/main/x-pack/plugins/fleet/server/services/elastic_agent_manifest.ts repo

[gizas]

Two more things needed for the story to close:

• To rerun another trigger of the CI (now that is merged in main) and check that any new changes related to folder /deploy/kubernetes will only be triggered only when merged to main
• To bypass CLA checker Update kubernetes templates for elastic-agent kibana#139630 - done

[gizas]

CLA fixed here: elastic/kibana#139630

[gizas]

Upon PR creation the Sync K8 stage is skipped
https://fleet-ci.elastic.co/blue/organizations/jenkins/elastic-agent%2Felastic-agent-mbp/detail/PR-1049/5/pipeline

And upon merging in main we can see that relevant stage 'Sync K8' was triggered and PR opened
elastic/kibana#139833
(Relevant jenkins Job here https://fleet-ci.elastic.co/blue/organizations/jenkins/elastic-agent%2Felastic-agent-mbp/detail/main/399/pipeline/2140)