Pass `app.yaml` config to the adapter

[jameslounds]

I intend on making a PR to implement this, but figured an issue first for thoughts would make sense.

I'm using this for a project, and I need to create an extra script to run after building to add some extra stuff to the app.yaml - some of these things are variable like env_variables, or service (dev, staging or prod).

I think it would be useful for the adapter to accept another argument, which will be dumped as yaml into the resulting app.yaml

kit: {
  adapter: appengine('build', [], {
    service:  process.env.SERVICE ,
    instance_class: "F4",
    automatic_scaling: {
      min_instances: 1
    },
    inbound_services: [ "warmup" ]
  })
}

[HalfdanJ]

Currently the script does enable this by adding a app.yaml file to the root. The app.yaml gets merged with the generated app.yaml file. But i have also thought about this solution, and I'm open to adding that as another option