fix: resolve build failure without async feature (fixes #52)

[hiking90]

Change rsbinder-aidl default features from ["async"] to [] to prevent async code generation when users don't need it.

Changes:

• rsbinder-aidl: Set default features to empty (sync-only by default)
• rsbinder-aidl: Remove unused sync feature
• rsbinder: Remove unused sync feature
• rsbinder: Add explicit features = ["async"] to build-dependency
• tests, example-hello: Add explicit features = ["async"] to build-dependency
• rsbinder-aidl/README.md: Update documentation with async/sync usage examples

This ensures that users who disable async in rsbinder won't get async code generated by rsbinder-aidl, preventing compilation errors like "cannot find type BoxFuture" or "could not find async_trait".