Set up Durable Functions workload CI/CD

Stand up the public and official/signed-build pipelines for the Durable
workload so it builds, packs, and publishes to the workloads feed
independently of the core CLI release cadence.

## Definition of Done

- [ ] Public-build pipeline runs on PRs touching the durable workload.
- [ ] Official/signed-build pipeline publishes the durable workload nupkg
to the workloads feed on release.
- [ ] Pipeline ownership documented (CODEOWNERS or pipeline description).
- [ ] A test release produces an installable package consumed by
`func workload install durable`.

Start by locating the existing core CLI release pipelines and the Durable workload packaging entry point, then compare how public and official builds are handled. Done means PR builds run for Durable changes, a signed release publishes an installable nupkg to the workloads feed, ownership is documented, and `func workload install durable` consumes a test release.

Written by the indexing model from the issue text.