Optionally eliminate blocking runtime.await calls by converting functions to coroutines.

Interop parallelism requires needs awaiting on results. Blocking awaits are bad for performance. TFRT supports lightweight resumption on threads, and coroutines are an abstraction than can be used to lower the kernels onto TFRT threads.

Reviewed By: ezhulenev

Differential Revision: https://reviews.llvm.org/D106508