feat: Allow new datastore implementation to be used

[grimly]

This PR removes the limitations of engines to use by OpenFGA.

Description

The /datastore/engine property get its enumeration restriction removed. Additionally, a description is added to document some engines are supported.

Enables the addition to OpenFGA of a new engine without the requirement of maintaining this chart again to use it.

DOES NOT add full support for additional engines as it will be the responsibility of the chart user to add the necessary configuration (ie: does not manages persistence of a sqlite file storage)

References

Fixes #171 by allowing the use of the sqlite engine name.
This cancelled PR ( openfga/openfga#2005 ) adds support for MSSQL, if this support is added later on, it would also benefit from this PR.

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