feat: allow global in helm values to allow openfga as dependency chart

[juusujanar]

Allow global property in values schema to be able to use the chart as dependency chart.

Description

This PR adds a global property to openfga Helm chart values schema file. This allows using the Helm chart as a dependency chart and provide values to it.

References

Fixes #106

Review Checklist

• I have clicked on "allow edits by maintainers".
• I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• The correct base branch is being used, if not main
• I have added tests to validate that the change in functionality is working as expected

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: juusujanar / name: Janar Juusu (a0aad26)
• ✅ login: jon-whit / name: Jonathan Whitaker (7bb22f7)