feat!: support of Spring Boot 4

[tgrafford]

BREAKING CHANGE: library no longer works with Spring Boot 3

This PR closes issue #59.

🔩 What code changed, and why?

Spring Boot upgraded to version 4.0.3 (latest as of writing). This included:

• Upgraded Gradle to v8.14.3
• Use the renamed module dependencies from Spring Boot 4
  • This introduced some minor refactoring related to interface and package structure of classes used
• Introduce testing dependencies for each module included
• Make use of JSpecify null-safety (introduced in Spring Boot 4)
  • This also included some minor refactoring where possible null-values was not handled as expected

👍 Definition of done

• opa-spring-boot is now compatible with Spring Boot 4

👟 How to test

The testing assumes you already have a Spring Boot application using opa-springboot and the application has been upgraded to Spring Boot 4 but does not compile.

1. Edit version in gradle.properties to version=2.0.0-LOCAL
2. Run ./gradlew clean build publishToMavenLocal
3. Edit dependencies in your Spring Boot application to use opa-springboot version 2.0.0-LOCAL
   Note: Ensure your application resolves dependencies from your local maven registry.
4. Build your application
5. (Optional) Run and test your application, either using ./gradlew bootRun or ./mvn spring-boot:run depending on build tool used.

⛓️ Related Resources

• Spring Boot 4.0 Migration Guide

[philipaconrad]

@tgrafford Thank you for the PR! I haven't looked at this repository in a long while, and only have done occasional maintenance on it since the original creator stepped away. I don't work actively in Java these days, so I strongly appreciate that you took the time to do a version upgrade like this.

This repo needs a few Github Actions upgrades and bugfixes to go in before this PR will go green on CI, which I'm working on presently. Once those are in, I'll review the PR, and we should be able to go from there.

Apologies for not noticing the PR sooner-- my Github notifications are flooded at all times, so this PR slipped through the cracks.

[philipaconrad]

I'm not particularly familiar with Spring Boot bits, but the OPA-related side of the changes seem fine, and the nullable annotations look good as well. I'm marking approve, so that we can finally get this set of changes rolled in. Thanks again @tgrafford!

[philipaconrad]

ℹ️ Rebasing the PR through the Github Web UI to catch up with main before merging...