Refactor newrelic-client-go to leverage tutone

[zlesnr]

Objective

Currenlty, the one place we are generating code from nerdgraph is in the newrelic-client-go project. Tutone as a project was spun out of the work done there, and so we should refactor the efforts to use tutone in place of what is happening when make generate is run.

Once the work in #12 is done, this should be somewhat simpler to replace the command in make generate to execute tutone and remove the go:generate statements from the relevant packages.