Allow configuring an alternative file as the `app.yaml`

The docs note that [the user may choose to rename their `app.yaml` files](https://cloud.google.com/appengine/docs/flexible/java/configuring-your-app-with-app-yaml):

> You can specify a unique name for your app.yaml files, but then you must specify the file name with the deployment command as well. For example, if you name your app.yaml file service-name-app.yaml or app.flexible.yaml, […]

We could allow the user to "bless" a particular file as the `app.yaml` (called the _service deployment descriptor_ in the docs) via a right-click option, and we store and retrieve this information somehow.

Or we could allow _Deploy to App Engine_ to be an right-click option for .yaml files that seem to encode a service deployment descriptor files.