Updates to match cedar#1014

[khieta]

Issue #, if available:

Description of changes:

The promised PR to get the Java bindings working with the upcoming FFI edits in cedar#1014.

Summary:

• Deletes the Slice class and related classes
• Adds new custom serializers for PolicySet and TemplateLink
• Updates the validator to use a PolicySet instead of Set<Policy> (which doesn't support templates or template links)
• Switches to using EntityUID in LinkValue instead of the (redundant) EntityTypeAndId
• Removes some (now unused) code from CedarJavaFFI and fixes some warnings

Note: CI failures are expected until cedar#1014 is merged. I plan to merge that PR first.

[khieta]

Update: cedar changes have been merged, so this PR is now ready for review 🎉