Real-time Policy Improvements and Clarifications

[schnuerle]

Explain pull request

Updating the MDS spec to make it clearer and easier to use Policy for real-time management of the public right of way.

Currently includes the following changes:

1. Real-time language clarification and examples
2. Noting that agencies may manage their public Policy feeds, for example with an API key
3. Policy uniqueness note to prevent identical policy duplication
4. last_updated parameter to return only the last updated date
5. active_only parameter to return only current, active policies
6. A Policy object relationship diagram.

To possibly come in the future:

1. Additional relevant policy examples (we may move these to an external document for ease of updates)
2. A push endpoint option (likely in a separate PR after this one)
3. A new Message/Notification/Communication endpoint/API (may come in a separate PR)
4. Adding a max_update_interval field like in Requirements to note how frequently the file may update (some discussion in WG that this may not be a good idea)

Is this a breaking change

• No, not breaking

Impacted Spec

Which spec(s) will this pull request impact?

• policy

Additional context

See Issue #906 for more history and context.