The purpose of this issue is to track changes made between betas. It might be that they have been done because some pending pull-requests have not made it to beta1 or that we found a much better way to find a given problem, before releasing to 4.1. These changes have no reason to be in the CHANGELOG so we'll use this issue.

beta1 to beta2

1. TransportInterface added and TransportFactoryInterface changed ([Messenger] Add TransportInterface as first class citizen sender+receiver #27164 from @nicolas-grekas)
2. ReceiverInterface's $handler now receives the Envelope object, not the message directly ([Messenger] Re-introduce wrapped message configuration (with fix) #27182 from @ogizanagi)