build(helm): sign helm chart releases

[burningalchemist]

This pull request updates the Helm chart release workflow to add support for PGP signing of chart packages and to improve the release process to only run on pushes to the master branch. It also adds Helm chart annotations for ArtifactHub signing metadata.

Helm chart signing and release process improvements:

• The GitHub Actions workflow (.github/workflows/helm-workflow.yaml) now installs the chart-releaser tool, imports a PGP signing key, and packages Helm charts with signatures when pushing to the master branch. The release step is updated to only run if packaging and signing are successful, and to skip re-packaging.
• Workflow permissions are updated to allow writing to repository contents, which is required for chart releases.

Helm chart metadata updates:

• The Helm chart (helm/Chart.yaml) now includes artifacthub.io/signKey annotations with the PGP key fingerprint and URL, enabling signature verification on ArtifactHub.