feature: exclusion/inclusion of plugins via additional metadata on services/routes

Users have expressed a need for two things:
- Ability to apply a global plugin but exclude certain services/routes (https://github.com/Kong/kong/issues/5233)
- Ability to define plugins templates and then directly use them on services/routes

The scope here is to figure out a way to define these things using decK's state file and then decK does the heavy lifting in the background to render the correct state and sync it to Kong.

The issue names no files, tests, or entry points. Start by examining decK's state-file handling and how Kong services, routes, and plugins are represented; done requires a decided metadata design that supports global-plugin exclusions and reusable plugin templates, then renders and syncs the correct Kong state.

Written by the indexing model from the issue text.