Add fallback_value option to Field

[danajackson2]

Fixes #4020. This adds a parameter called :fallback_value to Field, which allows you to specify a value to be returned if the usual ways (calling the method on an object, or checking for the key in a hash) fail. It's useful if there is a query that returns objects that might or might not have a certain field.

The motivation for this is a scenario where we have a data model that has lots of entity types. All of them have an id property. Most of them have a name property, but some of them don't. We want all of them to implement an interface (Node) that has a required id property and an optional name property.

interface Node { id: ID! name: String }

The default for the :fallback_value param in the initializer is :not_given, which allows us to set nil as the fallback if we want. The most common use case is probably with interfaces, so that's where I wrote tests. Are there any other places this should be tested? I added documentation to guides/fields/introduction.md under Field Resolution. Does that seem sufficient? I added a blurb to the error message about supplying a fallback_value to implement the field.

[rmosolgo]

This looks great, thanks for making this improvement! I pushed a commit to test the hash_key: ... behavior, although changes upstream mean there are actually several hash-lookup-related branches in field resolution. I wouldn't be surprised if there are few resolution paths (@dig_keys comes to mind) where fallback_value isn't properly applied yet. But we can cross those bridges when we get to them. Thanks again!