Align EvaluationError and ExtensionFunctionLookupError with cedar#745

[cdisselkoen]

Description of changes

Implement the suggestions in #745 for EvaluationError (and the previously-not-public ExtensionFunctionLookupError, which it references)

Issue #, if available

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):

• Does not update the CHANGELOG because my change does not significantly impact released code.

(covered by existing changelog entry)

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.)

[khieta]

Looks good. Left some minor comments & questions.

[khieta]

Looking at this again, I think ExtensionFunctionLookupError might have been previously public-ish due to the pub use cedar_policy_core::extensions in api.rs -- is it possible to remove the pub use there now?

[cdisselkoen]

I think that we don't need the extensions module public at the moment, because the Extensions type doesn't appear anywhere in our public API signatures. I've pushed a commit removing the extensions module from the cedar-policy API.