Dac: add data_streamer interface

Context

Discussion: https://hackmd.io/28mChZkRSrKn58jlS26Bmg

Adds a common base interface for Dac's Data_streamer.

This is a rough interface of the data streamer api for publishing and subscribing to root page hashes. This interface is defined so that dependent work can be unblocked. It communicates something close to the expected interface although admittedly rough around the edges (like this and this). This module is expected to be iterated on but not deviate too much.

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