[SPARK-16018][SHUFFLE] Shade netty to load shuffle jar in Nodemanger

## What changes were proposed in this pull request?
Shade the netty.io namespace so that we can use it in shuffle independent of the dependencies being pulled by hadoop jars.

## How was this patch tested?
Ran a decent job involving shuffle write/read and tested the new spark-x-yarn-shuffle jar. After shading netty.io namespace, the nodemanager loads and shuffle job completes successfully.

Author: Dhruve Ashar <dhruveashar@gmail.com>

Closes #13739 from dhruve/bug/SPARK-16018.