Scoped secrets and reusable workflow improvements [Public Preview]

### Value Prop
Scoped secrets let you control exactly where secrets are used, like specific repositories, branches, or workflows. This means reusable workflows get the secrets they need automatically, without extra setup or duplication, making secret management simpler and more secure.

### Expected Outcome
By introducing scoped secrets, teams can reduce manual work and avoid spreading secrets across multiple places. Organizations gain better visibility and control over secret usage, supporting safer automation and helping meet compliance and security needs.