feat: expose username and password environment variables

[aballet]

Description

This PR exposes the OPENFGA_DATASTORE_USERNAME and OPENFGA_DATASTORE_PASSWORD environment variables. They can be references directly in the values file, or through a secret. The username and password keys in the secret are also configurable. I took inspiration from Keycloak's way of accessing username and password from an existing secret.

References

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: aballet / name: Adrien Ballet (247d4f0)
• ✅ login: rhamzeh / name: Raghd Hamzeh (08f6672)

[MisterTimn]

I made similar changes to our fork or the openfga chart and made a PR before actually checking this one that's been parked here for quite a while.
#163

Hoping to bump this PR for review by code owners and would also propose to make it so that uriSecret gets the same configurability with regards to key selection, perhaps consolide them into one secret.