override fulcio server secret keys

[saisatishkarra]

Description of the change

1. Override fulcio server secret keys to avoid duplicating the secrets generated in standard TLS format generated by tools like cert-manager
2. Make the Private Key password as optional since not all secrets are generated to support it.

Existing or Associated Issue(s)

Fix to partially overcome: https://github.com/sigstore/scaffolding/issues/546

Additional Information

Checklist

• [x] Chart version bumped in Chart.yaml according to semver. Where applicable, update and bump the versions in any associated umbrella chart
• [x] Variables are documented in the values.yaml and added to the README.md. The helm-docs utility can be used to generate the necessary content. Use helm-docs --dry-run to preview the content.
• [ ] JSON Schema generated.
• [x] List tests pass for Chart using the Chart Testing tool and the ct lint command.

[Hayden-IO]

Cc @vaikas

[saisatishkarra]

@cpanato I haven't verified this change against the existing instance!! can you point me to docs to test it out?

[saisatishkarra]

@haydentherapper @cpanato please update the next course of action items if any changes are to be made/tested against to move this forward.