Support Java Functions Framework

[benjaminhuo]

We need to add OpenFunction async runtime support for https://github.com/OpenFunction/functions-framework-java and https://github.com/OpenFunction/builder, as well as add an unified function signature for both sync and async runtime

[benjaminhuo]

Fixed by OpenFunction/functions-framework-java#1 and OpenFunction/builder#58