Added PolicyId::new()

[aaronjeline]

Description of changes

Adds PolicyId::new(), a total function for constructing policy ids.
I will file a separate PR that moves the FromStr implementation to use Infallible, but that's a breaking change so wanted it separable.

Issue #, if available

551

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.

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