Automated testing of bundle apps

[yulianedyalkova]

• [ ] Bundle apps can get tested both standalone (via https://github.com/giantswarm/roadmap/issues/3271) and as part of a bundle.
• [ ] Bundle tests live in the bundle repo (this is the first point of integration and also how we deploy for customers).
• [ ] A command on the app repo PR should be able to automatically bump the app version in the bundle and run bundle tests.

### Tasks
- [ ] https://github.com/giantswarm/roadmap/issues/3340
- [ ] Trigger apps on bundle repo: same structure as app testing; can get triggered by a PR

[yulianedyalkova]

Current bundle repos and codeowners:

• https://github.com/giantswarm/security-bundle -> Shield
• https://github.com/giantswarm/service-mesh-bundle -> Cabbage
• https://github.com/giantswarm/auth-bundle -> BigMac
• https://github.com/giantswarm/karpenter-bundle -> Phoenix
• https://github.com/giantswarm/observability-bundle -> Atlas
• https://github.com/giantswarm/external-dns-bundle -> Phoenix