Add cmd to framework

Commands are represented in tnvme as a C++ classes in tnvme/Cmds directory. For fundamental details of commands refer to https://github.com/nvmecompliance/tnvme/wiki/Commands.

Details on how to add a cmd to framework.

Create a C++ class in tnvme/Cmds directory with the name of the class being the cmd name. For example if the cmd you are adding is Async Event Request Cmd to framework, then the name of the class in the tnvme/Cmds directory should be AsyncEventReq and the files which contain these classes are names as asyncEventReq.cpp and asyncEventReq.h. All the commands are derived from the base class called Cmd.