Add support for annotations without explicit value

[john-h-kastner-aws]

Description of changes

Adds support for annotations like @foo as sugar for @foo("").

Will require a small DRT update to account for the change in data structures. Will also require a small change to the docs to note that this syntax is also allowed.

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).
• [x] 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.)
• [ ] 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.
• [x] 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.)
• [ ] 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.)

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.
• [x] 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-docs. PRs should be targeted at a staging-X.Y branch, not main.)
• [ ] I'm not sure how my change impacts the documentation. (Post your PR anyways, and we'll discuss in the comments.)

[john-h-kastner-aws]

Though I'd like to propose a new terminology. Can we call what's after @ the annotation name and what's inside () the annotation argument.

I don't particularly want to pull a change in terminology into this PR.