Entity name setter method

[olivier-thatch]

By default, the entity names generated by grape-swagger in the OpenAPI schema are based on the fully qualified Ruby class names, e.g. MyAPI::Entities::Foo becomes MyAPI_Entities_Foo in the schema.

It is possible to customize entity names by adding an entity_name class method to the entity class (cf. docs).

However, doing this with Sorbet is very verbose:

class Link < Grape::Entity
  extend T::Sig

  sig { returns(String) }
  def self.entity_name
    "LinkedStatus"
  end
end

or even worse, if you're using the Style/ClassMethodsDefinitions Rubocop rule with EnforcedStyle: self_class:

class Link < Grape::Entity
  class << self
    extend T::Sig

    sig { returns(String) }
    def entity_name
      "LinkedStatus"
    end
  end
end

This PR adds an .entity_name= setter method to Grape::Entity to make it possible to define custom entity names in a much terser manner:

class Link < Grape::Entity
  self.entity_name = "LinkedStatus"
end