Add haf-app-test-runner shared image for HAF applications

Cherry-picked from main branch (MR !124 was incorrectly merged to main instead of develop).

Adds:
- Dockerfile.haf-app-test-runner: Ubuntu-based test runner with Python 3.14, Poetry, Docker CLI, JMeter, PostgreSQL client
- Build job in .gitlab-ci.yml
- Bake target in docker-bake.hcl