Use Protobuf for DRT

[cdisselkoen]

• Changes DRT to use Protobuf, rather than the undocumented JSON format, for communicating isAuthorized and validate requests from Rust to Lean.
• Adds a new DRT target testing that various structures properly roundtrip from AST to Protobuf and back.
• Updates the Lean Cli executable to expect Protobuf-encoded inputs for isAuthorized and validate (still expects JSON for other targets, like validateEntities and validateRequest).
• Removes tests for the Lean Cli executable, which used hardcoded JSON inputs. Tests for protobuf functionality, with protobuf inputs, were added in Cedar protobuf structures #485 (although they don't test the Cli specifically).

[john-h-kastner-aws]

Looks good, though I can't vouch for the correctness of the FFI code

[cdisselkoen]

Integration-tests CI failure is due (at least in part) to needing cedar-policy/cedar#1365