feat: template datastore uri secret

[jagregory]

It's useful to be able to template the secret ref for the datastore uri.

Situation where it's helpful:

1. You want to use a secret for the URI, not setting the uri/value directly
2. You may not know the secret name ahead of time, it's being set externally as a value
3. You're packaging openfga as part of an umbrella chart, and want to abstract configuration of openfga/you don't want users of your umbrella chart to have to know the structure of the openfga chart to be able to change database settings.

Description

Add a call to tpl before setting the environment variable, which allows variable substitution.

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