Extract `func quickstart` content from the core CLI into a workload

Move the curated content currently embedded in `func quickstart` out of
the core CLI binary and into a dedicated quickstart workload. This both
slims the core CLI and validates the workload model for non-runtime,
curated content. Release pipeline for the workload is tracked separately.

## Definition of Done

- [ ] Quickstart content lives in a new `Workloads.Quickstart` project.
- [ ] Core CLI no longer ships quickstart payloads inline.
- [ ] `func quickstart` resolves content from the installed workload.
- [ ] Existing quickstart commands and tests continue to pass.
- [ ] Workload manifest validated by the schema in CI.