add additional env variables to fulcio chart

[Racer159]

Description of the change

Support GCP credentials for external cloud provider workloads and add additional env vars for specifying values like VAULT_ADDR

Existing or Associated Issue(s)

This is a repickup of https://github.com/sigstore/helm-charts/pull/530

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.