Support native monitoring scripts

[amanjpro]

Currently monitoring jobs (scripts) are written in any language, and they run in a different UNIX process (using Java Process API). This is great because it enables a wide range of programming/scripting languages to write monitoring jobs, it makes it possible for DevOps/SRE to use their favourite languages for monitoring.

But, Java Process API is not all that speedy, so if a check is heavy, this adds even more to the runtime. For example a simple aws s3 ls can take upto 4 sec, a simple hdfs dfs -ls takes a few seconds for JVM to start. To avoid this, it would be nice to be able to write those checks in Java, and load them (as a PlugIn) to the same JVM that runs Greenish. So, users will have option to use PlugIn when speed is required, and use scripts when desired.