add RestrictedExpression::new_entity_uid()

[cdisselkoen]

Description of changes

Adds a new constructor to go with the other similar ones

Issue #, if available

Fixes #350

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 Dafny model or DRT infrastructure.

Disclaimer

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[cdisselkoen]

Downstream DRT failure is because cedar-policy/cedar-spec#156 is not merged yet. Merging this