Feature request: get executor

[llucasshenrique]

Request:

Create an get executor and migration to add it to projects using '@nx-go/nx-go'.

What it will do:

The executor will run go get on the application or library "projectRoot".
The migration will be used to add the 'get' executor to projects that are using '@nx-go/nx-go'.

Why we need:

Currently to get an package we need to navigate to each project folder and run go get package this create a problem when we want to split our code in various libraries.
With it will be simpler to install the dependencies to our projects with nx run project:get package, or nx get project package.

Current known workarounds:

#107 (comment)