Deny unknown fields in FFI. (#815)

[andrewmwells-amazon]

Description of changes

Deny unknown fields in FFI.

Issue #, if available

Resolves #815

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):

• Does not require updates because my change does not impact the Cedar formal model or DRT infrastructure.

[khieta]

I think you'll want to add #[serde(deny_unknown_fields)] to several more types here (e.g., PartialAuthorizationCall and TemplateLink).

You might as well just add this annotation to every struct defined in ffi/. Some of these structs are expected to be output rather than input (e.g., Response), but it doesn't hurt to error on unknown fields anyways.

[khieta]

Made the changes requested above (with @andrewmwells-amazon's permission) and merged with the new changes in #1014. Read for re-review!

[khieta]

Fyi cedar-drt failure is not due to this PR. cedar-java PR failure is caused by this PR -- looks like the Java wasn't generating completely well-formed JSON, despite passing all prior test cases. That's why it's useful to make this change 😄

Edit: fix for Java in cedar-java#178