Support identifiers as context decls in human-readable schemas

[shaobo-he-aws]

Description of changes

Issue #, if available

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.

[john-h-kastner-aws]

Is this fixing #681?

[shaobo-he-aws]

Is this fixing #681?

Yes, thanks for pointing it out.

[john-h-kastner-aws]

Do we need to make the same change for entity type attributes?

[aaronjeline]

LGTM

[khieta]

Lgtm. Left one small comment about test cases.

[shaobo-he-aws]

Do we need to make the same change for entity type attributes?

Yes, will do it in another PR. Actually, we don't need the same change because entity type attributes can be arbitrary types, including common types.