Add E2E test scenarios for JS source map upload

[fractalwrench]

Goal

Adds mazerunner scenarios that verify the BAGP can upload JS source maps in a react native app. This additionally integrates bugsnag-source-maps into the plugin as a prerelease version is now available.

Changeset

• Made it possible for each RN fixture to enable productFlavors, to enable Hermes, and to disable bugsnag's uploadReactNativeMappings flag
• Integrated the bugsnag-source-maps command by finding the project's node_modules folder and running the CLI
• Added scenarios for:
  • Assembling an app
  • Bundling an app
  • Assembling an app that uses product flavors
  • Manually invoking the source map upload task
  • Disabling sourcemap upload
  • Uploading a Hermes sourcemap
• Updated CI version of Node to 12

Steps were added to mazerunner to help verify the source map request contained the expected fields.

[fractalwrench]

Addressed review feedback