feat: support custom labels, annotations and external mounts

[alix-graylog]

Support custom labels, annotations and external mounts on all k8s objects.

Summary

Short description of the change.

Details

• List of meaningful technical changes

Linked issues

This fixes #??

PR Checklist

Please check the items that apply to your change.

• Tests added/updated
• Documentation updated
• This PR includes a new feature
• This PR includes a bugfix
• This PR includes a refactor

Testing Checklist

Static Validation

• Linter check passes: helm lint ./charts/graylog
• Helm renders local template sucessfully: helm template graylog ./charts/graylog --validate

Installation

• Fresh installation completes successfully: helm install graylog ./charts/graylog
• All pods reach Running state: kubectl rollout status statefulset/graylog
• Helm tests pass: helm test graylog

Functional (if applicable)

• Web UI accessible and login works
• DataNodes visible in System > Cluster Configuration
• Inputs can be created and receive data

Upgrade (if applicable)

• Upgrade from previous release succeeds
• Scaling up/down works correctly
• Configuration changes apply correctly

Specific to this PR

• describe what was specifically tested

Notes for reviewers

• Verify all applicable tests above pass
• Validate that the linked issues are no longer reproducible, if applicable
• Sync up with the author before merging
• The commit history should be preserved - use rebase-merge or standard merge options when applicable