Add Action Cable command callbacks at the connection level

[palkan]

Summary

This PR adds support for command callbacks to ActionCable::Connection::Base. For example,
the before_command callback is invoked before a command is processed by a channel. The after_command and around_command callbacks work similarly.

Other Information

The main motivation behind this PR is to make working with multi-tenant apps easier. Most multi-tenancy libs (apartment, acts_as_tenant to name a few) provide an API to set the current tenant for the block. To make sure that channels code is executed within a particular context, we had to either add smth like Tenant.switch! tenant before every action (too error-prone) or patching the #dispatch_websocket_message method (better, but hacky):

class Connection < ActionCable::Base::Connection
  def dispatch_websocket_message(*)
    using_current_tenant { super }
  end
end

I believe, we need an official API for that (and other similar use cases). With the proposed API, the example above could be rewritten as:

class Connection < ActionCable::Base::Connection
  around_command :set_current_tenant

  private

  def set_current_tenant
    using_current_tenant { yield }
  end
end

[jorgemanrubia]

This is great, thanks @palkan.

cc @jeremy

[jeremy]

Nice integration point for instrumentation as well. Thanks @palkan!