Extended Field DSL is not available within Function

[RabidFire]

As written in the docs, a GraphQL::Function is a reusable container for field logic. However, extended Field DSL (specified using GraphQL::Field.accepts_definitions(...)) is not available within Functions.

Example

# Extend the Field DSL to accept :custom_metadata_key
GraphQL::Field.accepts_definitions(
  custom_metadata_key: GraphQL::Define.assign_metadata_key(:custom_metadata_key)
)

# This works
field :myField do
  custom_metadata_key 'test'
end

# This does not work
class MyFunction < GraphQL::Function
  custom_metadata_key: 'test'
end

field :myField, function: MyFunction.new

The problem is that GraphQL::Function is quite different from GraphQL::Field, GraphQL::ObjectType, etc, in that they are not defined (and therefore does not include the GraphQL::Define::InstanceDefinable module) and are instead subclassed.

Proposed Solutions

1. (Workaround) Define a class method within the Function.
2. Hook into Field.accepts_definitions to automatically define_singleton_method on GraphQL::Function.
3. (Ideal) Wait for Object-oriented schema definition #820 - perhaps we won't need Functions if we can simply inherit from GraphQL::Field to define reusable fields.

[rmosolgo]

Function is out, Resolver is in, and Resolver should have all the API elements of normal class-based schema definition.