Create ExecuteProcess processor for NiFi-MiNiFi-cpp

Instrumenting application servers/devices with (Mi)NiFi agents gives ops teams the ability to add arbitrary system check and metrics.

NiFi proper's ExecuteProcess lets you use the wide variety of existing Linux tools and data gatherers, but MiNiFi-cpp lacks this today. Thus servers/containers need to be configured during build time (for example: https://github.com/randerzander/TechOps/blob/master/containers/base/scripts/nmon.sh), which is tedious and error prone compared to being able to have the MiNiFi agent invoke processes at will.

ExecuteProcess in MiNiFi-cpp would significantly improve the value of the agent for server/container instrumentation.