Add context argument to Serializer initializer

[MoskitoHero]

Description

This MR adds the possibility to define a custom context object to pass to the initializer of the Serializer.

This change also allows the context to be available in nested one or many association blocks inside the serializer definition.

So now you can do

class ProductSerializer < Barley::Serializer
  attribute :name do
    object.localized_name(context.locale)
   end

  many :storages do
    many :shelves do
      object.localized_name(context.locale) # context is available here too!
    end
  end
end

Type of change

Please delete options that are not relevant.

• [x] New feature (non-breaking change which adds functionality)
• [x] This change requires a documentation update

How Has This Been Tested?

• [x] updated the serializer_test.rb file with relevant tests

Benchmarks

Barley is fast, and should be kept as fast as possible. Provide benchmarks related to speed and memory footprint.

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing tests pass locally with my changes