Add Entity Slicing using Entity Manifests to Cedar

[oflatt]

Description of changes

This PR allows users of Cedar to get an entity slice of an existing Entities store using the entity manifest. Mainly this is useful for testing entity manifests. However, it may also be useful for users to load all entities in to memory, but only send a slice over the network to a server.

Later, I re-implemented most of this code in #1208. You can squash this code with all the PRs on top of it into one giant PR, or just review this and then review the replacement.

PR stack: https://github.com/cedar-policy/cedar/pull/1102 (Merged) https://github.com/cedar-policy/cedar/pull/1105 (This PR) https://github.com/cedar-policy/cedar/pull/1154 https://github.com/cedar-policy/cedar/pull/1156 https://github.com/cedar-policy/cedar/pull/1171

1196

https://github.com/cedar-policy/cedar/pull/1208

Issue #, if available

Checklist for requesting a review

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

• [ ] A breaking change requiring a major version bump to cedar-policy (e.g., changes to the signature of an existing API).
• [ ] A backwards-compatible change requiring a minor version bump to cedar-policy (e.g., addition of a new API).
• [ ] A bug fix or other functionality change requiring a patch to cedar-policy.
• [ ] A change "invisible" to users (e.g., documentation, changes to "internal" crates like cedar-policy-core, cedar-validator, etc.)
• [x] A change (breaking or otherwise) that only impacts unreleased or experimental code.

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

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

• [ ] Does not require updates because my change does not impact the Cedar formal model or DRT infrastructure.
• [ ] 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.)
• [x] Requires updates, but I do not plan to make them in the near future. (Make sure that your changes are hidden behind a feature flag to mark them as experimental.)
• [ ] I'm not sure how my change impacts cedar-spec. (Post your PR anyways, and we'll discuss in the comments.)

[oflatt]

This PR stacks on top of #1102 It looks like I can't select that PR as a base branch since it's from my fork of Cedar.

[oflatt]

Looks like changing the API for cedar testing broke downstream dependencies build

[shaobo-he-aws]

Closed in favor of a single PR.