Tezt/Cloud: use a companion echo rollup for DAL scenario

What

Starts an echo rollup (!18491 (merged)) fetching all the slots produced during the scenario.

Why

Having a rollup will be used to assess the rollup infrastructure is or is not a bottleneck.

How

• Starts a rollup that follows all the slots from producers.

Manually testing the MR

Build the kernel as explained in !18491 (merged) (or simply make build-kernels), then run a scenario with --echo-rollup:

dune exec tezt/tests/cloud/main.exe -- DAL --localhost -v --tezt-cloud arch --website --grafana --producers 3 --stake 1,1 --echo-rollup

Once everything is running, connect to the echo-rollup-operator container (see http://localhost:8080/#debugging) and look at the kernel logs:

tail -f /tmp/tezt-<...>/1/echo-rollup-operator-rollup-node/kernel.log

For example with three producers, you should be able to see logs of the form:

Retrieved page 0 for level 13, slot index 1 successfully. 3967 bytes read
[...] // logs for slot 1
Retrieved page 0 for level 13, slot index 2 successfully. 3967 bytes read
[...] // logs for slot 2
Retrieved page 0 for level 13, slot index 3 successfully. 3967 bytes read
[...] // logs for slot 3

and even some non attested slots from time to time, depending on your setup (it can happen on my leptop).

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