Add Rails component configuration

[delner]

This PR depends on #809.

As a follow-up to #450 and #764, this pull request adds block style configuration for Rails components available, to make configuration more expressive and customizable.

Some new configuration examples include:

# Customize ActiveRecord with block
Datadog.configure do |c|
  c.use :rails do |rails|
    rails.service_name = 'my-app'

    rails.active_record do |active_record|
      active_record.service_name = "#{rails.service_name}-postgres"
    end
  end
end

# Customize different ActiveRecord databases with block
Datadog.configure do |c|
  c.use :rails do |rails|
    rails.service_name = 'my-app'

    rails.active_record describes: :primary_db do |active_record|
      active_record.service_name = "#{rails.service_name}-primary-db"
    end

    rails.active_record describes: :secondary_db do |active_record|
      active_record.service_name = "#{rails.service_name}-secondary-db"
    end
  end
end

# Deactivate ActiveRecord
Datadog.configure do |c|
  c.use :rails do |rails|
    rails.active_record false
  end
end

The Rails configuration now allows all its sub-integrations (rack, active_record, action_view, action_pack, active_support) and their constituent options to configured in this manner.