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 #530

Additional Information

Checklist

• Chart version bumped in Chart.yaml according to semver. Where applicable, update and bump the versions in any associated umbrella chart
• 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.
• List tests pass for Chart using the Chart Testing tool and the ct lint command.

[Racer159]

On this though would it make sense to use a pattern more similar to Rekor for extra env vars? https://github.com/sigstore/helm-charts/blob/main/charts/rekor/templates/server/deployment.yaml#L105 - it may be nice in many cases to pull env vars from secrets rather than pass them through Helm values. (i.e. VAULT_TOKEN - this env var would likely ideally come through a secret or be an extra volume mount)

[Racer159]

(would also like to make a similar change to the rekor chart - could do that in this PR or a separate one - currently there is a way to force things to work but it isn't a great solution: https://github.com/defenseunicorns/uds-package-sigstore/blob/main/bundle/uds-bundle.yaml#L52)

[Sifungurux]

How do I vote up on this. Fulcio needs to set envs to get client_id and secret. Or can I pick up this somehow