Add support for search_index params in rekor chart

[cmurphy]

Follow the pattern used by trillian, which can either manage its own database container or connect to a GCP SQL instance. If a container is created, the trillian mysql image is reused for convenience.

Description of the change

Existing or Associated Issue(s)

Additional Information

Checklist

• [ ] 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.

[haydentherapper]

@bobcallaway @vaikas Any suggestions since y'all have done the most around the mysql trillian configuration?