Allow dev to disable certain code generation

[pauldemarco]

Use the build.yaml options section to disable certain generation from the tool.

For instance, users who do not need Firebase to/from can disable it by adding it like so:

targets:
  $default:
    builders:
      auto_data:
        options:
          create_firebase: false
          create_json: false

[pdf]

Should this be in the builder options for global control, or the @data annotation for per-model control, or both?