Sequencer: Add sequencer batcher

Context

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This MR adds a new implementation of batcher, which looks into kernel sequencer durable storage in order to get a number of messages in the delayed inbox.

For now, user's L2 messages might not be included in a batch, though this functionality is in the MR but has not been tested yet.

Also, optimistic state computation is not supported yet.

This one also abstract batcher events in sc_rollup_lib which should be backported.

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