DAL/Node: add a DAL daemon to fetch slots from neighbors

Context

The goal of this MR is to introduce a communication mechanism between DAL Nodes.

• We introduce a notion of neighbors which are hard-configured peers.
• DAL node follows monitor_slot_headers for each neighbor and fetches slots when published by downloading shards (one by one for now)

For now, the approach is naive.

• The DAL node downloads every slots by downloading the first 1/redundancy_factor of the shards
• We don't use L1 informations to know which shards a dal node is associated to

Manually testing the MR

make build-unreleased
dune exec tezt/tests/main.exe -- --file dal.ml --verbose --title 'Alpha: dal node slots propagation'

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