init: update the default sample Score file generated

[Abhishek9639]

This is a follow-up to score-spec/score-compose#476 and score-spec/score-k8s#302 as requested by @mathieu-benoit.

The default score.yaml generated by init was using a basic stefanprodan/podinfo example that didn't really showcase what Score can do. This PR replaces it with a more meaningful sample using scorespec/sample-score-app:latest with postgres, dns, and route resources, along with header comments pointing to the Getting Started
guide.

Since this is a stub/template implementation with placeholder provisioners, cross-resource output substitutions (e.g. ${resources.db.name}) are intentionally omitted they would fail at provisioning time since the stub always returns empty outputs.

Also switched from struct-based YAML generation to a DefaultScoreFileContent raw string constant (same approach as
score-compose and score-k8s), which is simpler and supports comments.

Updated TestInitNominal and TestInitAndGenerate_with_sample to reflect the new sample structure.

[Abhishek9639]

Hi @mathieu-benoit,
Raised this as a follow-up to the score-compose and score-k8s PRs. One small note since this is a stub implementation with placeholder provisioners that return empty outputs, I kept the cross-resource substitutions out of the sample to avoid provisioning errors.
Let me know if you'd prefer a different approach.
Thanks

[mathieu-benoit]

LGTM