Fixed #2016 issue - Allow referencing entities JSON files in run-test…

[mihai2500]

…s input

Description of changes

Added support for referencing external entity JSON files in the cedar/cedar-policy-cli/src/lib.rs (run-tests) .
Added support for referencing external entity JSON files in the cedar/cedar-policy-cli/tests/sample.rs (cedar run-tests) command instead of requiring entities to be defined inline in test JSON.
Changes:
Modified CheckedTestCaseSeed to track the test file directory for resolving relative paths;
Updated the deserializer to detect when entities is a string (file path) vs. inline JSON array;
Entity file paths are resolved relative to the test file's directory;
Added test case demonstrating the new functionality;

Issue #, if available

Fixed issue #2016

Checklist for requesting a review

The change in this PR is (choose one, and delete the other options):

• A bug fix or other functionality change requiring a patch to cedar-policy.

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.

I confirm that cedar-spec (choose one, and delete the other options):

• I'm not sure how my change impacts cedar-spec. (Post your PR anyways, and we'll discuss in the comments.)

I confirm that docs.cedarpolicy.com (choose one, and delete the other options):

• I'm not sure how my change impacts the documentation. (Post your PR anyways, and we'll discuss in the comments.)

[john-h-kastner-aws]

I think we want to do something here to protect against possible path traversal. We mainly see this being used as a development tools, but it'd be best to proactively prevent this in case it's called on untrusted input.