feat(argocd-image-updater): add commit signing support

[dlactin]

Added support for commit key signing via SSH and GPG key along with documentation updates

Relies on argoproj-labs/argocd-image-updater#710

Checklist:

• I have bumped the chart version according to versioning
• I have updated the documentation according to documentation
• I have updated the chart changelog with all the changes that come with this pull request according to changelog.
• Any new values are backwards compatible and/or have sensible default.
• I have signed off all my commits as required by DCO.
• My build is green (troubleshooting builds).

[yu-croco]

Hi @dlactin , thank you for your PR!
Since we follow upstream's manifest, so let us resume reviewing this PR when argoproj-labs/argocd-image-updater#710 is merged and released as specific version.

[mkilchhofer]

Seems that it isn't part of 0.13.0 or am I wrong?

• chore(argocd-image-updater): Update dependency argoproj-labs/argocd-image-updater to v0.13.0 #2692

[dlactin]

Seems that it isn't part of 0.13.0 or am I wrong?

* [chore(argocd-image-updater): Update dependency argoproj-labs/argocd-image-updater to v0.13.0 #2692](https://github.com/argoproj/argo-helm/pull/2692)

You're right, didn't quite make it into the 0.13.0 release. If the new cadence is followed I think I'll have to revisit in 3 months.