Add an optional parameter for CodeBuild Project runtime

From Nima: "would be great if we can make buildspec's phases/install/runtime-versions customizable. Currently whatever you put falls under phases/install/commands & phases/build/commands. I'm asking because I had a situation where I wanted to install a go package and I couldn't and installing go would increase build time significantly."

This seems like something we can handle with an optional parameter

Start by tracing the CodeBuild project configuration and buildspec generation to find where phases/install/commands and phases/build/commands are defined. Determine how an optional runtime-versions parameter should be passed through while preserving current behavior, then verify both the existing command flow and the requested Go installation case.

Written by the indexing model from the issue text.