more simplification of protobuf schema format (followups to 1488)

[cdisselkoen]

Description of changes

More simplifications of the Protobuf format for Schema, some suggested in #1488 and some based on conversations with @john-h-kastner-aws offline.

• resolving the TODO: Schema is just a list of EntityDecl and a list of ActionDecl, not a map, as this is simpler and sufficient
• removing the ability to have open attributes, as this is not a stable feature (in both entities and records)
• the context for an Action must be a record, so instead of Type, we require it to be a map<string, AttributeType>
• rename Type.Ty to Type.Prim which is more clear
• remove the Never, True, False, EmptySet, and AnyEntity types, as these should never appear in schemas
• remove the ActionEntity type as action entities can just be encoded as Entity type (we do not support action attributes in Protobuf, as that is not a stable feature)
• remove EntityRecordKind in favor of inlining it into Type
• a few field renumberings, which is a breaking change we can make now but not after stabilization

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):

• Updates the "Unreleased" section of the CHANGELOG with a description of my change (required for major/minor version bumps).

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.