Allow `String` to be used as the cursor type

[myronmarston]

The relay pagination spec says:

An “Edge Type” must contain a field called cursor. This field must return a type that serializes as a String; this may be a String, a Non-Null wrapper around a String, a custom scalar that serializes as a String, or a Non-Null wrapper around a custom scalar that serializes as a String.

Whatever type this field returns will be referred to as the cursor type in the rest of this spec.

ElasticGraph provides a Cursor type for use here. However, the spec allows other types to be used for the cursor. In an organization that used a custom scalar cursor type that was called something else, you can use type_name_overrides in your Rakefile to solve this:

  tasks.type_name_overrides = {
    Cursor: "RelayCursor"
  }

It's also valid for the cursor type to be String. But this doesn't work:

  tasks.type_name_overrides = {
    Cursor: "String"
  }

There are two problems here:

• This causes ElasticGraph to generate scalar String in the GraphQL schema...which conflicts with the existing definition of String.
• ElasticGraph has a scalar coercion adapter for Cursor which should not apply to String. (That is, it's logic is still needed for cursor values passed as a String, but not for other String values).

We'll need to find a solution for both of these. Maybe the cursor logic can be moved out of the scalar coercion adapter and applied to individual cursor values rather than to the Cursor type?