PBT for the serialization of the DAC payload

Context

This MR does two things:

• Factors out some helper functions in the existing test_dac_pages.ml
• Introduces simple PBT for round-trip (serialization/deserilazation) of the DAC payload posted via single request to the new stack based implementation.

As such this MR is one of the many towards the fully functional message aggregator that will be eventually PBT tested.

NEXT

As a next step there would be a follow up that would improve the existing Dac_payload_handler See the issues #4572 (closed).

Together with this we could also start to think about PBT test for serialization/deserialization roundtrip in case the original payload is received via multiple requests. I currently see two options.

• Factor out Dac_payload_handler (stack based impl of merkle shape serialization) to a separate module and pbt test its roundtrip of serializatin/deserialization in case the payload is received via multiple messages.
• Postpone this and wait for the new send message RPC and test this as a form of tezt integration test.

Closes #4549 (closed)

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR