Inconsistencies in foreign key reference format in deck

In routes, we use a dict to refer to a service (notice service: {name: svc1} )
```
services:
- host: mockbin.org
name: svc1
routes:
- name: foo
paths:
- /foo
service:
name: svc1
```

Same convention is followed for degraphql_routes.

However, plugins expect a string value, rather than a dict. The API accepts a dict (as checked with GW 3.8) but deck format doesn't. This could be something we take care of in a major release for deck. The uniformity can help us with a few bugs that arise with foreign key resolution for plugins. It's a good UX as well.

The issue names no files, tests, or entry points; begin by locating plugin foreign-key parsing and deck schema handling. Done means plugin references accept the same dict form as routes and degraphql_routes, with the resulting foreign-key resolution behavior made consistent.

Written by the indexing model from the issue text.