Description of changes

Makes two more changes to the protobuf format:

1. changes the request context from Expr to map<string, Expr>, since it's required to be a record anyway. This aligns with the format for entity attributes which is also map<string, Expr>.
2. in PolicySet, changes both templates and links from a (map from id to object) to (just a list of object), since the objects in question already contain the ids. This makes the encoding faster and more compact.

Issue #, if available

Checklist for requesting a review

The change in this PR is (choose one, and delete the other options):

• A change (breaking or otherwise) that only impacts unreleased or experimental code.

I confirm that this PR (choose one, and delete the other options):

• Does not update the CHANGELOG because my change does not significantly impact released code.

I confirm that cedar-spec (choose one, and delete the other options):

• Requires updates, and I have made / will make these updates myself. (Please include in your description a timeline or link to the relevant PR in cedar-spec, and how you have tested that your updates are correct.)

I confirm that docs.cedarpolicy.com (choose one, and delete the other options):

• Does not require updates because my change does not impact the Cedar language specification.