Add support for descriptions for `Ash.Type.Enum` and `Ash.Type.NewType` types

[sevenseacat]

Is your feature request related to a problem? Please describe.

It's possible to add a description when defining an Ash.Type.Enum, eg.

defmodule AshGraphql.Test.StatusEnum do
  use Ash.Type.Enum, values: [:open, :closed], description: "A status field"

(I don't see this in the docs anywhere for Ash.Type.Enum, but it doesn't raise an error...)

This description should flow through to any generated GraphQL schema documentation.

I know we use Absinthe under the hood, and Absinthe supports @desc for enums so we might be missing a step when converting Ash types to Absinthe types?

It's not currently possible to add a description to types defined with Ash.Type.NewType, but there should be a PR open for that soon. When that is added, those descriptions should flow through too.

[zachdaniel]

Types don't currently have a description option built in, and it's hard to say for sure that we want to add one to all types for display in all cases given the varied use cases of types. However, we can add a callback to the AshGraphql.Type module that is graphql_description/1 (takes the constraints). And then when generating types with Ash.Type.NewType and Ash.Type.Enum we can check if it's defined and use that to describe the type. So basically the same solution for enums and new types.

PRs welcome!