Add a "register_runtime" method to the mlir.execution_engine and show calling...

Add a "register_runtime" method to the mlir.execution_engine and show calling back from MLIR into Python

This exposes the ability to register Python functions with the JIT and
exposes them to the MLIR jitted code. The provided test case illustrates
the mechanism.

Differential Revision: https://reviews.llvm.org/D99562