feat: add example application

[jimmyjames]

Description

Adds a simple example application that uses the FGA Spring Boot Starter. Features:

• Uses an in-memory database and loads two document objects on application startup (document:1 and document:2).
• Creates a new store and simple authorization model on application startup, along with a single tuple granting user:anne the viewer relation to document:1.
• Exposes an endpoint (/documents/{id}) which uses the fga bean in a @PreAuthorize expression to ensure that the user (hard-coded to anne in this example) has the can_read relation to the requested document. If the FGA check returns true, the document is returned to the caller. If the FGA check fails, it will result in a 403.
• Also exposes an endpoint to create a document, which will write an authorization tuple to FGA defining user:anne as having the owner relation to the document.

This change does not add any tests; https://github.com/jimmyjames/fga-spring-examples has some great examples of writing tests and using the FGA test container; we'll create another issue to do something similar here (may be a good opportunity for a community contribution 😄).

Closes #5

References

#5

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

[jimmyjames]

Fossa checks failing, likely false warnings re. licenses, will need to review them.

[jimmyjames]

Due to some fossa license issues with hibernate (uses LPGL), using spring-data-jpa and its h2 is likely going to be problematic (until/if hibernate switches to apache 2.0 as discussed). Closing this PR to rework to remove those dependencies, and also because this PR uses the multi-module structure which we removed due to nexus plugin issues.