add resource reference field on paths

[rambleraptor]

x-aep-resource establishes a list of parents. The individual API endpoints do not establish which fields correspond to a parent resource (if any). This could be established through some guessing, but it's better to annotate these fields specifically.

This introduces x-aep-resource-reference to indicate that a particular field represents a parent resource. This is super useful for the Resource Explorer in particular.

🍱 Types of changes

What types of changes does your code introduce to AEP? Put an x in the boxes that apply

• [x] Enhancement
• [ ] New proposal
• [ ] Migrated from google.aip.dev
• [ ] Chore / Quick Fix

📋 Your checklist for this pull request

Please review the AEP Style and Guidance for contributing to this repository.

General

• [x] Basic Guidance is met.
• [x] Ensure that your PR references AEPs correctly.
• [x] My code has been formatted (usually prettier -w .)

💝 Thank you!