Introduce global attachments and global errors

[epszaw]

The latest versions of Allure Report supports global attachments and global errors entities, which can be created in context of entire report, not a specific test result.

Global attachment:

Global error:

Usage samples:

# frozen_string_literal: true

require "allure-rspec"

AllureRspec.configure do |config|
  config.results_directory = "allure-results"
  config.clean_results_directory = true
end

RSpec.configure do |config|
  config.before(:suite) do
    Allure.add_global_attachment(
      name: "suite-start",
      source: "Created from before(:suite) in the sample project.",
      type: Allure::ContentType::TXT
    )
  end

  config.after(:suite) do
    Allure.add_global_error(
      message: "Sample global error",
      trace: "Created from after(:suite) to demonstrate *-globals.json output."
    )
  end
end

[andrcuns]

Changes look good to me, just one very minor point about redundant passing of default value

[epszaw]

Hey, @andrcuns! Thank you for the review. Accepted the proposed changes, the tests have passed