Tsa secret optional for tuf

[cvegagimenez]

Description of the change

Make the TSA secret reference optional for TUF chart.

Existing or Associated Issue(s)

#735

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.
• [x] JSON Schema generated.
• [x] List tests pass for Chart using the Chart Testing tool and the ct lint command.

------------------------------------------------------------------------------------------------------------------------
 Charts to be processed:
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 tuf => (version: "0.1.13", path: "charts/tuf")
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"sigstore" already exists with the same configuration, skipping
Linting chart "tuf => (version: \"0.1.13\", path: \"charts/tuf\")"
Checking chart "tuf => (version: \"0.1.13\", path: \"charts/tuf\")" for a version bump...
Old chart version: 0.1.12
New chart version: 0.1.13
Chart version ok.
Validating ~/Workspace/helm-charts/charts/tuf/Chart.yaml...
Validation success! 👍

Linting chart with values file "charts/tuf/ci/ci-values.yaml"...

==> Linting charts/tuf
[INFO] Chart.yaml: icon is recommended

1 chart(s) linted, 0 chart(s) failed

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 ✔︎ tuf => (version: "0.1.13", path: "charts/tuf")
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All charts linted successfully```