One of the goals of the new SDK is to provide clear documentation + convenient helpers to let communities develop their own extensions around the SDK. And the Integrable module added in this PR is our first step to make that happen.

The integrable module provides a simple DSL for registering the extension. Let me use sentry-rails as an example:

module Sentry
  module Rails
    extend Integrable
    register_integration name: "rails", version: Sentry::Rails::VERSION
  end
end

Once the extension is registered, 2 things will happen:

1. It generates Sentry::Rails.capture_exception and Sentry::Rails.capture_message methods.
2. It also generates the SDK meta for the extension, {name: "sentry.ruby.rails", version: Sentry::Rails::VERSION} in this case.

Then in the extension gem, all the reporting should be done via the newly generated helpers. This is because:

• Those helpers will inject { integration: "integration_name" } to the event hints. So the users can later identify the source of each event.
• Events created from those helpers will have the integration meta as their SDK information.
• In the future, we might also introduce more advanced integration-based features that need to be triggered by these helpers.

This change also fixes the current SDK-info overridden issue, which is when using sentry-rails and sentry-sidekiq are used together, the SDK info will be decided by the order of the gem definitions in the Gemfile instead of where is the event being created.

But besides the above bug fix, this change shouldn't affect normal users.