Update google omniauth generator to use Rails Credentials/ENV variables for APP ID and Secrets

[coolprobn]

Right now we add a line for the App ID and Secrets like this and ask users to figure out the addition of those values.

config.omniauth :google_oauth2, "APP_ID", "APP_SECRET"

Instead we should take these values from Rails credentials or ENV variables; whatever users prefer.

You can see a similar implementation for this in sentry generator:

boring_generators/lib/generators/boring/sentry/install/install_generator.rb

Line 35 in 68133b1

def sentry_dsn_key

Also need to add tests for this change.