adds upsert_entities

[a0js]

Description of changes

Adds upsert_entities to the Entities implementation. Internally, it removes any entities that match the uid and performs an add.

Issue #1479

Checklist for requesting a review

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

• A backwards-compatible change requiring a minor version bump to cedar-policy (e.g., addition of a new API).

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):

• Does not require updates because my change does not impact the Cedar formal model or DRT infrastructure.

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.

[a0js]

This still requires iterating through all entities per updated entity in remove_entities which may not be the best alternative. I'm just not sure where to add an update capability.