refactor ext_hotprofiler.cpp into new file ext_hotprofiler.h

Summary: Following existing coding conventions, split out a .h file from ext_hotprofiler.cpp.  In this way, 3rd party profilers can be written and loaded as DSOs.
Closes #3027

Reviewed By: @ptarjan

Differential Revision: D1401272

Pulled By: @JoelMarcey