Add FFI and JNI support.

Native Interop features/issues needed

• Published versions
• jnigen without build

FFI essential features

• Add FFIgen config generator
• Add logic for handling every type crossing between platform and dart for FFI
• Add custom class support
• Add enum support
• Add Codec for untyped data
• Add synchronous Host Api method support
• Add Async Host Api method support
• Add synchronous Flutter Api support
• Add Async Flutter Api support

JNI essential features

• Add JNIgen config generator
• Add logic for handling every type crossing between platform and dart for JNI
• Add custom class support
• Add enum support
• Add Codec for untyped data
• Add synchronous Host Api method support
• Add Async Host Api method support
• Add synchronous Flutter Api support
• Add Async Flutter Api support

After

• Stay updated to latest versions of ffi tools
• Fix various Dart optional/named parameter options
• Significant code cleanup
• Address ergonomics of multi-generation process
• Write docs and how-to
• Significant reviews and adjustments based on those reviews
• Address significant performance issues related to large classes

FFI

• Fix SwiftFunction annotation

JNI

• Rewrite config generator to use Dart instead of YAML
• Rewire error handling to allow for better error information