Fix tokio_async disabled build; Support bringing your own import libraries

[cgettys-microsoft]

• Fix tokio_async disabled build by conditionally compiling out some more async related code
• Add new sample library to ensure we're able to compile with default features disabled. Not doing the full 2^3 combinations, but right now, the 3 are completely orthgonal as far as I know, so this is a good start
• add new default-on bundled_lib option; if disabled, consumer will need to provide import libs in their own build script or similar. This allows using custom import libs. It's not pretty, but should work. For now, only address windows bring own scenario.