use PolicyId when constructing policies

[khieta]

Description of changes

• Changed the public Policy::parse and Template::parse APIs to use an Option<PolicyId> instead of Option<String> to set the policy id
• Changed the internal parse_policy* APIs similarly
• Updated tests to use PolicyId::new instead of PolicyId::from_str because it's prettier

The diff is largely just the result of updating test cases. The only interesting edits are in:

• cedar-policy-core/src/parser.rs
• cedar-policy/CHANGELOG.md
• cedar-policy/src/api.rs
• cedar-policy/src/api/id.rs
• cedar-policy/src/ffi/utils.rs

Issue #, if available

Resolves #1049

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).

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):

• 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.)

[cdisselkoen]

LGTM