Draft: Scoru node: receive and publish messages to L1

Context

Implements the batcher for the SCORU node. The batcher takes signed messages and aggregates them before sending them to the L1. For now, the batch is aggregated and sent at each new head of the L1. What remains to do for a follow-up MR:

• take account of rollbacks (cache the last two sent batches according to the finality for example). This could be implemented using the injector from TORU node.
• decide (or leave it for later) the maximum size of batches, and if the batcher should send a unique manager operation or multiple manager operations.

Implements #2748 (closed)

Based on !4938 (merged) and !5025 (merged)

Owner: @mebsout

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