[base-controller] Create MessengerClient superclass for BaseController and stateless non-controllers

[MajorLift]

Explanation

Some of our classes that were previously labeled as controllers have empty state, making them non-controllers.

As part of Wallet Framework team's Q2 2024 OKRs (O3KR1), we're currently in the process of upgrading these non-controllers to use the messenger pattern without inheriting from BaseController.

To minimize redundant code, the reused portion of BaseController should be extracted into a new MessagingEndpoint (tentative) class that serves as a parent class to BaseController and all stateless non-controllers, and includes the logic a class needs in order to accept and use a messaging system.

We should ensure that both MessengerConsumer and BaseController are compatible as base classes for the polling-controller mixins.

References

• See Non-controller classes with empty state should not inherit from BaseController #4432
• Followed by [base-controller,polling-controller] Make non-controller superclass compatible with polling-controller mixins #4503