helm: create a Helm chart for apm-server binary

[axw]

Motivation/summary

WIP, experimental Helm chart for deploying APM Server standalone in Kubernetes, either as a Deployment+Service, or as a DaemonSet.

Each pod gets a volume for its data directory, defaulting to emptyDir but configurable through Helm values. The Helm chart generates a self-signed TLS certificate and a random secret token. The chart's values currently enable tail-based sampling by default for testing/demonstration purposes; this would be switched to disabled later.

Checklist

• Update CHANGELOG.asciidoc
• Update package changelog.yml (only if changes to apmpackage have been made)
• Documentation has been updated

How to test these changes

1. Clone the PR
2. Create an Elastic Cloud deployment (for Elasticsearch & Kibana)
3. helm install <name> ./helm --set elasticsearch.host=https://....es.elastic-cloud.com:443 --set elasticsearch.username=elastic --set elasticsearch.password=<password>

Related issues

#2107

[mergify]

This pull request does not have a backport label. Could you fix it @axw? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.

NOTE: backport-skip has been added to this pull request.

[v1v]

run docs-build