Update GraphqlInputObject compiler to skip methods from code

[jeffcarbs]

Motivation

We've come across some graphql input objects that define a method with the same name as an argument. For example:

class SomeInput < ::GraphQL::Schema::InputObject
  argument :foo_id, String

  sig { returns(Integer) }
  def foo_id
    self[:foo_id].to_i
  end
end

In reality, what happens is that the graphql gem meta-programmatically defines a foo_id method (source) and then the user-defined foo_id method overrides it. If the sigs are different (like in the example above) this confuses sorbet so we should avoid defining RBI for any arguments where an explicit getter is already defined in code.

Implementation

I updated the GraphqlInputObject compiler to only generate RBI for methods defined via the graphql gem.

Tests

See added tests.