Add before_execute and after_execute callbacks

[yuki24]

Use cases

• They are useful for application-level logging and analytics
• before_execute is specifically useful when e.g. needing to assign a header to every request on the fly
• after_execute is specifically useful when e.g. needing to raise an exception when the HTTP status is 5xx (could be user's choice)

Exmaples

class Metaphysics < Artemis::Client
  before_execute do |document, operation_name, variables, context|
    # do stuff before making a GraphQL request
  end

  after_execute do |data, errors|
    # do stuff after receiving a GraphQL response
  end

  ...
end