CMakeLists.txt for executor_runner

Summary:
A simple build system that should make it easier for partners to cross-compile, and to integrate into their non-buck2 build systems.

This uses the `extract_sources.py` tool from D47979069 to avoid hard-coding lists of source files, pulling them from the buck2 graph instead.

D48019443 updates the shipit config to copy CMakeLists.txt to the root in github.

Reviewed By: tarun292, larryliu0820, cccclai

Differential Revision: D47927863

fbshipit-source-id: b9a66af540c2df79743f926cce30794a57b63b4a