Proto: Scoru: return level when commitment was first published in Sc_rollup_publish

Context

A commitment can be cemented only after sc_rollup_challenge_window_in_block levels have passed since when the commitment was originally published.

In order for the rollup node to determine whether a commitment can be cemented, it will need to know the level when the commitment was originally published. This MR tweaks the Sc_rollup_publish manager operation to return this information when a commitment is published by the rollup node (This is fine as we require a rollup node to publish a commitment before cementing it, since the staker of the rollup node will need to be staked on the commitment to be cemented).

Related: #2878 (closed)

Manually testing the MR

Unit tests:

dune exec src/proto_alpha/lib_protocol/test/unit/main.exe \
      -- test "^\[Unit\] Sc_rollup_storage.ml$"

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