exec-next: Add method: to dataload shorthands

[OuYangJinTing]

Motivation

A very common pattern in our schema is resolving a scalar by first batch-loading an association, then calling a method on the loaded associated object.

Without this PR, every such field requires a custom def self.<field> batch resolver. For example, in our UserType we end up with a lot of boilerplate like this:

# app/graphql/types/user_type.rb
field :age, String, null: true, resolve_batch: true

def self.age(objects, context)
  dataload_all_associations(context, objects, :verified_info).map { it&.age }
end

field :country, String, null: true, resolve_batch: true

def self.country(objects, context)
  dataload_all_associations(context, objects, :latest_device).map { it&.country }
end

def self.dataload_all_associations(context, records, association_name, scope: nil)
  source = if scope
             context.dataloader.with(GraphQL::Dataloader::ActiveRecordAssociationSource, association_name, scope)
           else
             context.dataloader.with(GraphQL::Dataloader::ActiveRecordAssociationSource, association_name)
           end
  source.load_all(records)
end

This becomes repetitive quickly and makes the type file mostly boilerplate.

With the method: option on dataload: shorthands, the same fields can be written declaratively:

field :age, String, null: true,
      dataload: { association: :verified_info, method: :age }

field :country, String, null: true,
      dataload: { association: :latest_device, method: :country }

It keeps the dataloader batching, removes the need for one resolver per field, and makes the schema definition a lot more readable. Because method: works for with:, association:, and model: shorthands, it covers most of our real-world cases.

Summary

• Adds a method: option to hash-style dataload: field configs under Execution::Next. When present, the loaded value's public method is called before returning it, e.g. dataload: { association: :author, method: :name } is equivalent to dataload_association(:author)&.name.
• Works with all three hash-style shorthands (with:, association:, model:).
• Updated guides/execution/next.md with examples for each shorthand.

Tests

Added specs covering method: for the source shorthand, the Rails association shorthand, and the Rails model shorthand.

[rmosolgo]

Thanks for this improvement! I agree this is a nice convenience.