support HTTP template with artifact in body

# Summary

Support artifacts HTTP template bodies

# Use Cases

Send a generated artifact to a rest api via a HTTP template.

```yaml
bodyFrom:
artifact:
name: …
```

---

**Message from the maintainers**:

Impacted by this bug? Give it a 👍. We prioritise the issues with the most 👍.

The issue provides no file or test path. Start by locating HTTP template handling and existing artifact support; done means an HTTP template can populate its body from the named artifact and send it to a REST API, with coverage for the YAML shape shown.

Written by the indexing model from the issue text.